Implement caching and fast updates of the starred list

[Malabarba]

Every time we refresh, the list of repos starred by the user has to be fetched in batches of 100 (due to how the API works), but this can be a little slow if you have several hundred starred repos on Github.
It should be possible to cache the list of repos between sessions, and only update it by querying the API for repos recently added since last query.

This started as #102.