Add test vectors for message encoding

[pirapira]

This issue keeps track of considering the idea of adding test vectors for message encoding.

When this idea is implemented, the spec will contain for each message test cases for encoding and decoding.

A test case for encoding would look like:

LockedTransfer(field1=value1, ..., fieldN=valueN) with private key 0x....... is encoded as 0x........

A test case for decoding would look like:

The byte sequence 0x...... is decoded as LockedTransfer(field1=vaue1, ..., fieldN=valueN) signed by address 0x.......

@kelsos has a script that's useful for generating the encoding test vectors. https://github.com/kelsos/raiden-protocol-messages/blob/master/messages/messages.py