ENH: Esky support

[westurner]

https://github.com/cloudmatrix/esky/

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 capable of freezing apps with py2exe, py2app, cxfreeze and bbfreeze. Adding support for other freezer programs should be straightforward; patches will be gratefully accepted.