Persist User/Repo Information

[JoshVarty]

GitHubInformationRetriever.GetRepoInformation() and .GitHubInformationRetriever.GetUserInformation() are called every time a user or repository is browsed. This creates a request to GitHub.

We should persist this information and load it ourselves. We're probably going to need a database or datastore for this, though.

[AmadeusW]

The information is persisted as file on the hard drive:
./SB_Files/username/user.data
./SB_Files/username/repository/repo.data

Possible stale information is served immediately. At the same time, we are asynchronously fetching new information from github (user's avatar, website, repo's number of forks and stars) only if the file is out of date. Currently it's set to 2 hours in SourceBrowser.Site.Utilities.FileUtilities.FileIsFresh but we can tune it to once a day.