Add USB Serial Port Callbacks

These edits allow the functions lineCodingEvent and lineStateEvent to be used in sketches to know when the virtual serial port's control lines or baud rate changes.

They are needed for creating sketches that act as a USB->Serial Port interface or interfacing with software that uses these signals as controls.

They were originally authored by Peter Van Hoyweghen http://petervanhoyweghen.wordpress.com/2012/11/08/using-the-leonardo-as-usb-to-serial-converter/