Automated sketchbook libraries detection

As I stumbled in #50, I started using @mchughj solution ( mchughj@e55df22 ), for I needed a way to seamlessly use the libraries I had already in my sketchbook dir. I thought that, given the beauty of the autodetection principle of ino, I could add a fully-fledged and transparent sketchbook libraries detection. This solution works fine under OSX and linux, tested with Arduino ide 1.0.