Fix attributes for Sensor and Message classes
Fix attributes for Sensor and Message classes

* Add protocol_version attribute for Sensor class.
* Fix assigning payload as protocol_version at presentation.
* Change Message attributes type and subtype to integers at init. All
    Message attributes should be integers except the payload.
* Fix some minor PEP8 issues.

Br3nda/pymysensors,afeno/pymysensors,theolind/pymysensors