Possibility to track license changes

[Trilarion]

Once we can automatically detect licenses (#191), try to detect license changes automatically. Maybe sometimes, the license got more restrictive, for example going from CC without NC to CC with NC, etc. ...

[Trilarion]

One such possible case is #409.

We can automatically detect the license as one step and then checkout repositories commit by commit and look for the license. However that may take a long time for only a few cases. Maybe we can simply look in the index for typical license files and check them out as a non-complete but more efficient way.