Add the concept of commands to the msensor class.

Currently, commands are only supported by NXT I2C sensors. This eliminates
duplicate modes for sensors that have commands like "reset" or "calibrate"
that don't actually change the mode of the sensor.

Two new attributes are added to the msesnsor class:

* commands: (read-only) Returns a space separated list of supported commands or
  -ENOSYS if commands are not supported.
* command: (write-only) Sends a command to the sensor.