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.
I restored the original pin mapping and added a commented out I2C pin 
map to beaglebone.js.

I also updated the readme.

On 12/04/2014 02:47 PM, Julian Duque wrote:
>
> In lib/beaglebone.js:
>
> > @@ -20,8 +21,8 @@ var modesMapping = {
> >  var pinMappings = [
> >    "P8_7", // 0
> >    "P8_8", // 1
> > -  "P8_9", // 2
> > -  "P8_13", // 3 - PWM
> > +  "P9_20", // 2  IC2 SDA - using Leonardo Pinout
>
> My call is on not changing the mappings but adding the I2C pins in the 
> README, what do you think?
>
> —
> Reply to this email directly or view it on GitHub 
> <https://github.com/julianduque/beaglebone-io/pull/13/files#r21330681>.
>