Module for HMC5883L magnetometer

Finally!  Sorry for the previous noise, I'm still learning git and managed to stuff things up pretty good.

Here's a clean pull request to include a module for HMC5883L magnetometers (compass) communicating via I2C.

It has been tested on a BeagleBone Black and is based on code from:
 * http://think-bowl.com/i2c-python-libraries-for-the-raspberry-pi/
 * https://github.com/adafruit/Adafruit_HMC5883_Unified

The class was initially a copy of the ADXL345 module.

It should work on a Raspberry Pi without modification as the Adafruit_I2C module handles the hardware specific I2C stuff.