Base structure for WAMP messages processing and Tornado-related handler

Added features:
- WAMP `tornado WebSocket handler`
- `Extendible message` pattern for parsing and creating WAMP messages. It is currently applied to: `HELLO`, `WELCOME`, `ABORT` and `ERROR`
- Extendible `processor pattern`, which can be easily coupled to the WAMP requests handler. The handler delegate messages to specific classes, which are responsible for processing and defining outcomes. It is currently applied to: `HELLO` and `UNHANDLED`.

Regarding tests: 99% coverage
Should the previous connection be closed (if it exists)?