Add new index format to support localization and graphics

Merged username-removed-24982 requested to merge eighthave/fdroidclient:add-new-index-format into master

We have been talking about a new index format for a long time. Adding support for localization and graphics is the straw that broke the camel's back for me, so I finally dug in. I also took this opportunity to change the flow on both client and server side to take advantage of some really great libraries out there, like Jackson on the client side, and python-yaml and python-json on the server side.

There is a test repo here with the new index: http://testy.at.or.at/fdroid/repo/