Write light-client proofs for tendermint rpc

[ethanfrey]

We can actually prove all responses are tied to a header that is signed by the validator set, and quickly validate new headers. This code is working in golang in tendermint core https://github.com/tendermint/tendermint/tree/master/lite It would be interesting to expand this into our stack, so we can get these guarantees without trusting a proxy.

Cool stuff, but totally not on needed path for launch.