Added GNU GPLv3 License information.

Hi Thomas,

Alright, still not sure if I'm doing this the "right" way but here goes. This is a pull request from my Master (Head) into your master. It is nothing more than the inclusion of the standard GNU GPLv3 License document and standard header at the top of the Radiono sketch. I also changed one line at the top of Si570.cpp altering MIT to GNU GPLv3. I did compile and upload the sketch and it worked. So I'm not expecting any surprises. Surely nothing could go wrong with simple comment fields... :lol:

73, Steve.