Rework message serialization

Add new class 'MsgSerializable' subclassing the already existing
'Serializable' class.

Remove messagemap, the message strings are now a static attribute of
every message class.

Remove message_read, replaced by the classmethod
MsgSerializeable.stream_deserialize()

Remove message_to_str, replaced by MsgSerializeable.to_bytes()

Make classes in bitcoin.net subclasses of Serializable

Add tests for messages and net

Add examples/msg-serializable.py

Add test_suite to setup.py
Merge pull request #7

0e4e1b2 Rework message serialization (Florian Schmaus)