Websocket support

[martyall]

Here is the api documentation https://github.com/ethereum/go-ethereum/wiki/RPC-PUB-SUB

I have a feeling that this is going to end up looking something like multifilters because we only want to have one websocket connection open in order to be able to hook up arbitrary event monitors, which means somehow the type of the websocket is going to carry some open sum variant of all the possible messages which can come through.

I'm open to the design and discuss what the types might look like, but fair warning that this one is probably hard.