I2c sensors

This builds on top of the uart-sensors branch. UART and I2C sensors are modified to use the same msensor class.

Checkout the docs on the [wiki](https://github.com/mindboards/ev3dev/wiki/Using-Sensors).

To try this kernel without compiling, as root on the EV3:
```bash
wget -P /etc/apt/sources.list.d http://ppa.lechnology.com/ev3dev.list 
apt-key adv --keyserver keyserver.ubuntu.com --recv-key 1F8AF88C82F77C03
apt-get update
apt-get install ev3dev-base linux-image-3.3.0-ev3dev-00.01.02-dlech-g93df567
cp /boot/uImage-3.3.0-ev3dev-00.01.02-dlech-g93df567 /media/mmc_p1/uImage 
reboot
```