Issue 100: Add a settings menu with a single preference for controlling sync frequency.

I added a new 'Settings' menu to the Dashboard Activity which contains a single preference: the frequency, in minutes, of how often to synchronize repositories.

This works, but it's sort of a first pass at fixing this problem. Ideally, I think it would be nice to be able to sync with different options (e.g. every X minutes/hours, once per week, once a day, X day of the month, etc..). This is possible right now, but it requires that the user calculate the sync frequency to minutes, which is rather tedious for longer-term syncs.