Implement gettxout rpc. 

This pull request adds rpc server support for the gettxout rpc. Fixes issue #141. This pull request depends on https://github.com/conformal/btcjson/pull/27.
This should probably be `btcjson.ErrInvalidParameter`.  Also, please make the message consistent with decodeScript here https://github.com/conformal/btcd/blob/08c01f08b103f6d6875ae8464abf0f5a1c307c67/rpcserver.go#L995-L996.

Returning the data you received in the error is beneficial for callers since they can see in the error what they sent, which is quite helpful during debugging.  An unexpected error here would typically mean they aren't sending what they think they are.