Add tsl2561 light sensor

[mschlenstedt]

Add TSL2561 Light sensor, based on library from Adafruit. Config example:

sensor_modules:
- chip_addr: 0x39
  module: tsl2561
  name: tsl2561_1
  gain: 1
  integration_time: 13.7
sensor_inputs:
- digits: 4
  interval: 30
  module: tsl2561_1
  name: illuminance_tsl2561
  type: lux
  retain: true