Bug 611 - serialization of path attributes

Bug 611 - serialization of path attributes
- added Nlri,originatorId and clusterId serialization to BGPUPdateMessageParser
- added Ipv4NextHopCase Ipv6NextHopCase serialization
- added serialization methods to attribute objects
- attribute_flags and attribute_type added to all attribute serializers
- attribute descriptors <attribute type, attribute length, attribute
  value> unified by AttributeDescriptor interface
- added tests for Update message serialization

Change-Id: I49e6bb6c0883f125a24fa2a058f0f0bf0e404261
Signed-off-by: Martin Bobak <mbobak@cisco.com>