Adapt msgpack-rpc layer to the new architecture
Add synchronous msgpack-rpc layer

Previously the synchronous API was emulated by the `Client` class, now it fully
implemented in the msgpack-rpc subpackage
fix msgpackrpc init
Add `compat` module for helping with compatibility across versions
Rewrite API layer for the new architecture

The API layer now fully distinct from the msgpack-rpc subpackage. Every API
class is created with a Session instance for making msgpack-rpc requests.
Move script_host.py/plugin_host.py to 'plugins' subpackage
Refactor the initialization package to reflect the new API
Update tests
Add more python versions to travis and install flake8 plugins
Specify version/platform dependencies in setup.py