Cleanup PullRequestsFetcher and add new store strategy
Cleanup PullRequestsFetcher and add new store strategy

Going forward we will no longer keep closed pull requests around. I
thought this was necessary but we actually don't need any historic
data. Therefor we will just remove all pull requests before fetching a
list containing only open pull requests from GitHub.

substancelab/pullhub,substancelab/pullhub,substancelab/pullhub