Sensor: Allow user to set bus

[ddunmire]

At the moment the sensor class will assume i2c bus 1 for rpi. The bus should probably default to 1, but allow optional value to be passed in constructor and expose it as a property.

In my case I am using rpi zero w with a battery pack that causes issues with Atlas sensors on bus 1 issues. So I moved mine atlas to i2c bus 3.

[ddunmire]

possible workaround (but not optimum)
sensor = sensors.Sensor("temp", address=args.i2caddress) #TODO: add bus to sensor.
sensor.client.bus = args.i2cbus
sensor.client.device_file.close() # close device_file created by sensor constructor
sensor.client.open_file() # open new file using new bus number.
sensor.connect()

perhaps have sensor.connect verify valid device_file and/or call AtlasI2C.open_file()