Esky

* only delete files from the top-level app dir if they explicitly belong
to a version that is being removed.

* implemented preliminary support for freezing with py2app.
* added module esky.patch for diffing and patching frozen apps:
* provides a generic file format for diffing/patching directories.
* can recurse into compressed zipfiles, giving patches an order
of magnitude smaller than produced by naively applying bsdiff.
* individual files are diffed via bsdiff if cx-bsdiff is installed.
* bsdiff-based patches can be applied with no external dependencies.
* added support for differential updates in DefaultVersionFinder.
* added "bdist_esky_patch" distutils command for producing differential
updates in the format expected by DefaultVersionFinder.
* added filesystem-level locking to protect in-use versions from removal.
* added attribute Esky.active_version, which is non-None when the esky
refers to the currently-running application.

* some backwards-incompatible API changes:
* replace Esky.install_update(v) with Esky.auto_update().
* new utility methods Esky.install_version(v)/Esky.uninstall_version(v).
* rename Esky.fetch_update(v) to Esky.fetch_version(v)
* merge VersionFinder.prepare_version into VersionFinder.fetch_version
* made all VersionFinder methods take the target esky as first argument
(in preparation for differential update support in a later version).
* added support for freezing with cx_Freeze.
* improved support for freezing with py2exe:
* added a few well-known hacks to get it to play nice with common
third-party packages (win32com, wxPython).
* implemented a custom chainload() function to avoid using execv().
* added ability to set the icon on each executable, for freezer
modules that support it (currently py2exe and cx_Freeze on win32).
* added more version-handling utility functions: get_all_versions,
is_version_dir, is_installed_version_dir.
* made Esky.cleanup() catch and ignore common errors.
* added support for Python 3 (via distribute's "use_2to3" flag)
* added a brief tutorial and example application

* added ability to bundle MSVCRT as a private assembly (this is off
by default, since you must have a valid license to redistribute it).
* refactored to support multiple freezing backends.
* added support for freezing with py2exe.