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 think the error should be ignored here as in https://github.com/conformal/btcd/blob/08c01f08b103f6d6875ae8464abf0f5a1c307c67/rpcserver.go#L1000-L1002.  See comment below.

```Go
	// The disassembled string will contain [error] inline if the script
	// doesn't fully parse, so ignore the error here.
	disbuf, _ := btcscript.DisasmString(script)
```