Add beginIP() method to use static ip

This pull requests adds support for static ip using the ```set ip address <addr>``` command. The WiFly API has been extended by a beginIP() method that expects the ip adress as it's only parameter as a a zero terminated string.

Instead of ```WiFly.begin();``` you can now use ```WiFly.beginIP("<your-ip>");``` to initialize the module. This will disable dhcp and use the provided ip adress instead.