Automatically downloads/installs wiringPi library

Hello again!

I have changed the rebar.config file to automatically download and install the required wiringPi library, and created a Makefile that will do that and make rebar itself, just to make it easier to use, given that rebar's error when a dependency is not found is somewhat cryptic.

Please review if it interests you to merge that into your code.

Thanks for the great work on this library! And I hope I can contribute a bit.

Marco