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.
As a note, the behavior here does not look consistent with the behavior of the original gettxout RPC in Bitcoin Core in regards to invalid hashes being supplied since it appears Core will accept those invalid hash strings.

That said, I think this is better since it forces the caller to provide a well-formed hash string.  I'm a firm believer in returning errors when callers supply malformed data.  The fact that Core allows strings that don't even look like hashes to be specified and treats them like a zero hash is just bad behavior in my opinion.