Execnet

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- refine termination some more: CTRL-C and gateway.exit will
now try harder to interrupt remote execution. this
helps to avoid left-over ssh-processes.
- fix read-on-non-blocking-files issue probably related to jython only:
the low-level read on subprocess pipes may be non-blocking, returning
less bytes than requested - so we now loop.
- Windows/python2.4: fix bug that killing subprocesses would fail
- make RemoteError and TimeoutError available directly on execnet namespace

- fix some doc and test issues (thanks thm and ronny), add ssh_fileserver example
- update internal copy of apipkg
- always skip remote tests if no ssh specs given

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- generalize channel-over-channel sending: you can now have channels
anywhere in a data structure (i.e. as an item of a container type).
Add according examples.

- automatically close a channel when a remote callback raises
an exception, makes communication more robust because until
now failing callbacks rendered the receiverthread unusable
leaving the remote side in-accessible.

- internally split socket gateways, speeds up popen-gateways
by 10% (now at <50 milliseconds per-gateway on a 1.5 GHZ machine)

- fix bug in channel.receive() that would wrongly raise a TimeoutError
after 1000 seconds (thanks Ronny Pfannschmidt)

--------------------------------

- revamp and better structure documentation

- new method: gateway.hasreceiver() returns True
if the gateway is still receive-active. remote_status
now only carries information about remote execution status.

- new: execnet.MultiChannel provides basic iteration/contain interface

- new: execnet.Group can be indexed by integer

- new: group.makegateway() uses group.default_spec if no spec is given
and the execnet.default_group uses ``popen`` as a default spec.

- have popen-gateways use imports instead of source-strings,
also improves debugging/tracebacks, as a side effect
popen-gateway startup can be substantially faster (>30%)

- refine internal gateway exit/termination procedure
and introduce group.terminate(timeout) which will
attempt to kill all subprocesses that did not terminate
within time.

- EOFError on channel.receive/waitclose if the other
side unexpectedly went away. When a gateway exits
it now internally sends an explicit termination message
instead of abruptly closing.

- introduce a timeout parameter to channel.receive()
and default to periodically internally wake up
to let KeyboardInterrupts pass through.

- EXECNET_DEBUG=2 will cause tracing to go to stderr,
which with popen worker gateways will relay back
tracing to the instantiator process.

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* introduce execnet.Group for managing gateway creation
and termination. Introduce execnet.default_group through which
all "global" calls are routed. cleanup gateway termination.
All Gateways get an id through which they can be
retrieved from a group object.

* deprecate execnet.XYZGateway in favour of direct makegateway() calls.

* refine socketserver-examples, experimentally introduce a
way to indirectly setup a socket server ("installvia")
through a gateway url.

* refine and automatically test documentation examples

--------------------------------

* fix EXECNET_DEBUG to work with win32
* add support for serializing longs, sets and frozensets (thanks
Benjamin Peterson)
* introduce remote_status() method which on the low level gives
information about the remote side of a gateway
* disallow explicit close in remote_exec situation
* perform some more detailed tracing with EXECNET_DEBUG

--------------------------------

* make internal protocols more robust against serialization failures

* fix a serialization bug with nested tuples containing empty tuples
(thanks to ronny for discovering it)

* setting the environment variable EXECNET_DEBUG will generate per
process trace-files for debugging