Add I2C-Master Library support

This adds support for the I2c library mentioned here:
  http://dsscircuits.com/articles/arduino-i2c-master-library.html
and available here:
  https://github.com/DSSCircuits/I2C-Master-Library

Tested with ADXL345 I2Cdev driver as well as seperate ITG3200 and
proprietary devices.