JSON Pickler for BSON values (other than BSONObjectId)

[kryptt]

We need an implementation of:
play.modules.gjson.JSONPickler[reactivemongo.bson.BSONValue]

or pretty much for these:
BSONArray, BSONBoolean, BSONDateTime, BSONDocument, BSONDouble, BSONInteger, BSONLong, BSONNull, BSONString, BSONTimestamp, BSONUndefined.

BSONObjectID is already there. see:
https://github.com/kindleit/gkit/blob/master/play-gresource-mongo/src/main/scala/PicklerInstances.scala