Fix issue in HttpSerializer.formatError where exceptions with null "message"
fields (NullPointerException) were throwing another NPE
Add UniqueIdRpc for all UID related API calls
Add the /api/uid/assign endpoint to allow assigning UIDs for metrics, tag names and tag values via the HTTP API
Add TestUniqueIdRpc.java for unit testing
Add parseUidAssignV1() and formatUidAssignV1() serializer calls
Closes #79