Allow user to edit number of update history days in prefs.

Instead of using hardcoded 14 days.
Log total time spent/bytes fetched from network while d/ling repo index.
Merge commit 'refs/merge-requests/16' of git://gitorious.org/f-droid/fdroidclient into merge-requests/16
Merge commit 'refs/merge-requests/17' of git://gitorious.org/f-droid/fdroidclient into merge-requests/17