Bug-479: Implementation of Vendor-Information Object

-reference: http://tools.ietf.org/html/draft-ietf-pce-rfc7150bis-00
-added yang model of new object
-added abstract skeleton of Vendor-Information object parser/serializer
-PCReq and PCRep Messages models and parsers enhanced with list of VI objects
-introduced new Vendor-Information object parsers/serializers registry
-added junit tests

Change-Id: Icbf1ff79542cc2f7e5219de208ec015e31323730
Signed-off-by: Milos Fabian <milfabia@cisco.com>