Added I2C Support

Added I2C support. This should obviously be peer reviewed. I tested using an 8x8 Matrix LED display. Results are identical for Arduino and BBB. I also threw together a quick demo for the TMP102 temperature module, and results are identical for Arduino and BBB. 

I am not sure if it worth providing tests for the 3 i2c methods, since there is no pin manipulation to test.
Not sure what you thoughts on this might be.
My call is on not changing the mappings but adding the I2C pins in the README, what do you think?