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.
I'd log and return btcjson.ErrInternal if the output index is < 0 or the output itself is nil, since it should be valid at this point.  ErrInvalidTxVout does seem like the correct error for an output index greater than the actual number from the MsgTx.