Add some menu actions for gist activities.
Simplify adapter view creation.
Implement search in repository list.

TODO: Make this less naive by using the search API. At this point, the
Java bindings don't support the V3 search API, and its results are
inconsistent - for example,
https://api.github.com/search/repositories?q=gh4a+user:maniac103 yields
an empty result.
Show contributors with avatar and amount of contributions.