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've changed error handling to separately handle the case of the output being nil. 

About `c.Ouput` being a signed integer: i'm not sure. `GetTxOutCmd.Output` in `btcjson` was originally a [signed integer](https://github.com/conformal/btcjson/blob/master/jsoncmd.go#L3524). Similarly, in the rpc implementation in Bitcoin Core the tx [output index](https://github.com/bitcoin/bitcoin/blob/f0fc81a0832a8fa93c5d7e619e25f739d72b49f3/src/rpcblockchain.cpp#L373) is also a signed integer