DOC: start enstaller documentation.

We use sphinx for documentation, and added a skeleton of documentation
for our egg format. We've only documented the proxy/soft-linking stuff
for now.
DOC: EGG-INFO files now mostly documented.

We also added kenneth reitz templates to have an html that looks decent
by default.
DOC: tweak doc to address review comments.
Merge pull request #112 from enthought/document_egg_format

Document egg format