Ensure the last modification are getted

Otherwise we wont get the latest check-ins
Use a better algorithm to get the last commit
Embed label support inside the GitImporter.

We make sure the label are sorted by date before using them. Optimisation
on the client side of the SDK could prevent a proper ordering of the
labels.

Also added most needed utility for labels processing.
Converted InvalidOperationException as NullPointerException.
Merge remote-tracking branch 'origin/master' into label-support

Get the lastest fix done on the master branch so we do not diverge too much.