cache github API responses to fight the API rate limit

All Github API response are cached using localStorage. When the API request fails, data are loaded from the cache. If the cache key does not exist, question marks are returned for all values:

![...](http://f.cl.ly/items/1e3v2G0q2N3r1g37161u/Screen%20Shot%202013-08-27%20at%207.38.31%20PM.png)