Based on the rpc/tests/client_test.go, I extracted a nice interface to make all this rpc calling much easier and type safe for the clients. I am using this in the light-client repo, but as @ebuchman mentioned, it really belongs in tendermint.

I will extract this code, clean it up a bit, and prepare a mock version for testing as well. If it is merged in develop, I will remove the duplicate code from light-client (which originally came from my personal repo tenderize...)

@ebuchman let me know if this is something that can make it in 0.9?