Adaptive sync interval

[czechboy0]

Find a way to gradually slow down the sync interval when we're approaching the GitHub rate limit. So the sync interval set by the user is the minimum time between two syncs, but not the maximum - we might slow down to not hit the rate limit before the hour is over. Will become more important with multi syncer support and branch tracking, but would be good to know Builda is never blocked by just plain waiting for the rate limit to catch up.

[czechboy0]

Hopefully won't be necessary after #62 is done.