Pull latest pages of stargazers

[rviscomi]

If a repository's cache is stale, only pull the most-recent significant pages of stargazers.

For example, there are 205 stargazers in the cache. The next page load discovers that there are now 210 stargazers. So only the last 5 stars are significant (assuming no un-stars; we'll save that problem for another day). Since there are 100 stargazers per page, we only need to look at the second page, assuming that the entire contents of the first page are unchanged and already in cache.

For large repositories like twitter/bootstrap, this will save hundreds of API calls over the network. (1 or 2 requests for <100 new stars instead of 390 -- and growing).