Lnd-grpc

Updated proto files and added new methods (listUnspent, subscribeChannels). Cleaned code.

Removed clunky error handling. This will be left for upstream.

Also removed docstrings added in last release, as it's easier and more comprehensive to check lnd's own RPC documentation.

Changed add_invoice() so it will accept either (hex) r_hash_string or (bytes) r_hash.

Other small fixes.

Removes asynchronous methods until asyncio implementation.
Added better error handling for GRPC errors.
Fixed bugs in methods requiring optional parameters.