Introducing EPL

EPL (Emacs Package Library) is an abstraction on top of package.el, providing a sane API for common operations.

It's also intended to be compatible with all versions of package.el, but for now it just supports recent Emacs snapshot.  I'll backport it asap.  I pushed it in this early state to get your reviews and feedback.

TODO:

- [x] Fix ~~unit~~ ecukes tests
- [x] Backport to stable Emacs releases
- [x] Backport to Emacs 23
- [x] Test

Supersedes #50.