Add RegisterPseudonym

Also: if multiple arguments have the same type, you can bundle the types:

 `RegisterNym(nymId, serverID string) (string, error)`

You switched the parameter order: [OT_ME.cpp](https://github.com/Open-Transactions/opentxs/blob/01bcf4d/src/client/OT_ME.cpp#L392) - it should be `serverId, nymId`