Add public domain license handling

* recognize licenses with shortname 'Public domain', return 'pd' as license name
* split license parsing into its own class
* add some sanity tests for main class

Change-Id: I596696150d5c9b6a6c9209f0fdb1d6904b6fc433
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/117
Merge "Add public domain license handling"