Yaybu

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- Improved validation for scripts in cwd.

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- 66, fix ordering of permission dropping so that we can actually drop group
permissions.

- 65, add a umask option to Execute.

- 64, proper error handling when assigning to an invalid property.

- After parsing a shell command, verify that the command exists (searching on
PATH if needed).

- Make sure errors from Yay are wrapped in ParseError so they are presented
properly.

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- Support a ``running`` option for ``Service``. This is a command that should
return an exit code of 0 if all services are running. If used with a buildout
generated ``isotoma.recipe.cluster`` symlinked into ``/etc/init.d`` this
might look like this::

resources.append:
- Service:
name: cluster
running: /etc/init.d/cluster running

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- Fix handling of broken links when replacing them.
- Fix handling of broken links for Link remove policy.

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- If remote process dies before we SIGKILL it, swallow any errors.

- The RemoteRunner object can now set SSH settings in its ssh_options
dictionary. For example::

runner.ssh_options["IdentitiesOnly"] = "yes"

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- Better error handling. Proper detection of resource redefinition and of
undefined resource type use. Yay node graph is walked lazily so Yaybu
errors can access line numbers and other metadata. The resource bundle
is parsed and error checked before we even try and connect to server.

- Exit code ``254`` is now used when no changes have been made. ``253`` is
used for unknown errors. ``255`` is now a connection errror.

- Refactor template handling to allow ``{% extends "index.html" %}``. You can
refer to templates on the ``YAYBUPATH``, and you can override templates by
putting new templates higher up the ``YAYBUPATH`` search.

- Yaybu hackers now have additional ways of invoking the debugger, including
an equivalent to the pdb ``pm()`` and ``set_trace()`` methods..

- File creation is now more secure: permissions are set before content
is written.

- We now use Yay 0.0.41 as it has a cleaner API for injecting Python objects.