Remove the rest of minjson

We switched from defining widgets using JSON to using HTML in
6b0be7b2ba87e07b130b3956e5c02e1fc23b8d2b. This removes the rest of
minjson, fixing issues discovered in #64.
Merge pull request #65 from gratipay/remove-minjson

Remove the rest of minjson
Find and replace gittip, Gittip, gttp
Widgets now work with legacy gittip-* attributes