(WIP, dont-yet-merge) JSON-rpc call

Trying to get simulated calls into the JS and JSON-rpc apis... But have to figure out how to revert it, [probably here](https://github.com/o-jasper/cpp-ethereum/blob/jsonrpc_call/libethereum/Client.cpp#L178). `State::call` seems to need changing because the [evm object](https://github.com/o-jasper/cpp-ethereum/blob/jsonrpc_call/libethereum/State.cpp#L976) is gone early otherwise, it also needs to revert the `addBalance`.

If this goes wrong.. it messes up the state completely.. right?

(Also needs to be worked into `eth/main.cpp` and `neth/main.cpp`, and where-ever that JS api lives i think)
This won't even compile: std::String is incorrectly capitalised. Please only submit patches that actually compile and are properly tested.