Added preliminary PWM support for BeagleBone Black
fixed error message, missing parameter
added todo
removed debugging
moved i2c to devices. Only contains i2c for now, but really is more generic.

In particular, devices such as HD44780 can be supported different ways. This device specifically has I2C, SPI and GPIO variants. Same device interface, different comms underneath.
update docs with I2C, PWM and devices