(minor) add wb.Snak.equal()

Allows to check whether one Snak is equal to another. Compares the Snak's type and property
ID. In case of PropertyValueSnak, also the value will be compared. Added tests for checking
equality for Snaks constructed from JSON to their JSON donors.

Change-Id: I4ba67322564b1a30968f0f1181721d56b0fe599a
Merge "(minor) add wb.Snak.equal()"