Esky

* implemented lazy imports, to reduce the startup time burden on frozen
applications that must call esky.run_startup_hooks().
* moved "files_differ" function into esky.util.
* esky.slaveproc: don't kill the slave process if something goes wrong
with the monitoring process.
* py2exe: make sure bootstrapped code runs as __main__

* added "pre_freeze_callback" and "pre_zip_callback" options to bdist_esky
command. They provide a convenient way to hook the build process with
e.g. code signing routines.

* added "bootstrap-code" option to bdist_esky, allowing you to specify
custom bootstrapping code without having to put it in a module.
* added "esky.slaveproc" module, to support automatic teardown of slave
processes when their master process dies.
* this is currently used for simulated execv() under win32, and
for the helper process in esky.sudo.
* py2exe: don't bundle files by default, as this can cause issues with
some libraries (specifically win32ui from my experiments).

THIS RELEASE INTRODUCES A NEW ON-DISK FILE LAYOUT - SEE BELOW FOR NOTES

* moved all esky control files into an "esky-files" subdir, to avoid
clutter in the top-level version dir if/when additional files are added.
* there are compatability hooks for the old file layout, but they
will be removed in the 0.9.* series.
* therefore, if you have deployed applications using esky 0.7.* or
lower, you MUST deploy an update using the 0.8.* series before
migrating to any newer versions of esky.
* bootstrap-manifest.txt: always use forward-slashes in paths, even
when building for Windows.
* Esky.fetch_version() now accepts a "callback" argument, which will be
called with a dictionary of download status information. For those who
hate callback functions, you can also use Esky.fetch_version_iter() to
get an iterator over the download status information.

* py2app: ensure apps are always inside an "<appname>.app" directory,
so they will appear correctly in finder etc.
* esky.util: added deep_extract_zipfile() function to extract a zipfile
while ignoring any common directory prefix.
* esky.finder: allow downloaded zipfiles to have a containing folder
around the appdir content, to allow friendlier zipfile behaviour.

* Esky.cleanup_at_exit: don't fail if there are no installed versions,
just use the currently-executing script.
* be more careful about missing files when manipulating eskys that target
other platforms (e.g. upgrading a win32 esky on a linux box).
* ensure that fcntl is always available in the bootstrap env.
* updated tutorial to talk about generating patches.
* have all executables lock the version dir at startup, even if they're
not launched through the bootstrap env.