* Add a general mysensors component. This sets up the serial comm
with the gateway through pymysensors. The component also
contains a decorator function for the callback function of
mysensors platforms. Mysensors platforms should create a
function that listens for the node update event fired by the
mysensors component. This function should call another
function, that uses the decorator, and returns a dict. The dict
should contain a list of which mysensors V_TYPE values the
platform handles, the platfrom class and the add_devices
function (from setup_platform).
* Change existing mysensors sensor platform to depend on the new
mysensors component.
* Add a mysensors switch platform. The switch platform takes
advantage of new functionality from the the fork of pymysensors
https://github.com/MartinHjelmare/pymysensors, that enables the
gateway to send commands to change node child values.
* Change const and is_metric to global constants, in the mysensors
component and import const depending on the mysensors version
used.
* Change variables devices and gateway to global variables.
* Add some debug logging at INFO log level.
