drivers/tsl4531x: Add power modes; redesign

Changes include:
- Implementation of two power modes for the driver: low and high
- Redesign and API change to the description in tsl4531x.h
- Full documentation
- Changing file structure and implementation to fit best practices as
  described in
  https://github.com/RIOT-OS/RIOT/wiki/Guide%3A-Writing-a-device-driver-in-RIOT
- Including I2C addresses for the rest of the range