esky:  keep frozen apps fresh

Esky is an auto-update framework for frozen Python applications.  It provides
a simple API through which apps can find, fetch and install updates, and a
bootstrapping mechanism that keeps the app safe in the face of failed or
partial updates.

Esky is currently compatible with apps frozen using bbfreeze or py2exe. Adding
support for other freezer programs should be straightforward; patches will be
gratefully accepted.

The main interface is the 'Esky' class, which represents a frozen app.  An Esky
must be given the path to the top-level directory of the frozen app, and a
'VersionFinder' object that it will use to search for updates.  Typical usage
for an app automatically updating itself would look something like this:

    if hasattr(sys,"frozen"):
        app = esky.Esky(sys.executable,"http://example.com/downloads/")
        new_version = app.find_update()
        if new_version is not None:
            app.install_update(new_version)

A simple default VersionFinder is provided that hits a specified URL to get
a list of available versions.  More sophisticated implementations will likely
be added in the future, and you're encouraged to develop a custom VersionFinder
subclass to meet your specific needs.

When properly installed, the on-disk layout of an app managed by esky looks
like this:

    prog.exe                 - esky bootstrapping executable
    updates/                 - work area for fetching/unpacking updates
    appname-X.Y.platform/    - specific version of the application
        prog.exe             - executable(s) as produced by freezer module
        library.zip          - pure-python frozen modules
        pythonXY.dll         - python DLL
        esky-bootstrap.txt   - list of files expected in the bootstrapping env
        ...other deps...

The "appname-X.Y" directory is simply a frozen app directory with some extra
bootstrapping information produced by esky.  To freeze your app in such a
format, there is a "bdist_esky" command that can be used with a standard
distutils setup.py file.

To upgrade to a new version "appname-X.Z", esky performs the following steps:
    * extract it into a temporary directory under "updates"
    * move all bootstrapping files into "appname-X.Z.platm/esky-bootstrap"
    * atomically rename it into the main directory as "appname-X.Z.platform"
    * move contents of "appname-X.Z.platform/esky-bootstrap" into the main dir
    * remove the "appname-X.Z.platform/esky-bootstrap" directory
    * remove files not in "appname-X.Z.platform/esky-bootstrap.txt"
    * remove the "appname-X.Y.platform" directory

Where such facilities are provided by the operating system, this process is
performed within a filesystem transaction.  Neverthless, the esky bootstrapping
executable is able to detect and recover from a failed update should such an
unfortunate situation arise.

To clean up after failed or partial updates, applications should periodically
call the "cleanup" method on their esky.