Adds support for batch operations in electrum RPC

[junderw]

Fixes #77

Instead of sending one object per line { ... }.

The client can send an array of objects [{ ... }, { ... }, { ... }, { ... }] and the responses will be sent back in batches.

[junderw]

Return early with errors was messing things up. So instead, every error turns into a json Value.

@mononaut if you can think of anything else that needs fixing, let me know.