Installation with **pip** now possible.

This set of modifications makes it possible to install *python-ebay* with *pip* (and also *easy_install* I think). 

The changes to allow the new installation method itself are rather small. Most effort went into a release script (`release.py`), that deletes and backups the key file, and also handles the password and user ID of PyPi. The documentation was adapted and slightly expanded. The documentation was also converted to RST markup, because PyPi needs it.

The changes were not rigorously tested, but both installation methods worked for me with a `virtualenv` Python. Installation with *pip* also worked with my system Python. (Both on Linux.)

I'll merge the changes, if no one objects, in a week or so.