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.
Please use a `btcutil.Amount` here.  We've been trying to make sure all new code uses the monetary type for monetary values.

`btcutil.Amount(mtx.TxOut[c.Output].Value).ToUnit(btcutil.AmountBTC)`