Update Change number of drops in Identity class
Fixed  Identity constructor should set QblPrimaryKeyPair #128
added registration of QblPrimaryKeyPairTypeAdapter in IdentitiesTypeAdapter
Update List to Collection.
add addDrops()
getDrops() returns a immutable collection
Merge pull request #148 from k-c13/identity-drop-key

Fixed  #109 (Update Change number of drops in Identity class) and #128 (Fixed Identity constructor should set QblPrimaryKeyPair)
Merge pull request #153 from aBothe/pr

Don't return null when being unable to deserialize a DropMessage's data field