USB serial baud arg type

Make USB Serial_::begin() function take an unsigned long argument for
consistency with HardwareSerial.

Signed-off-by: Paul Brook <paul@nowt.org>
USB CDC two argument begin()

Add two argument form of Serial_::begin

Signed-off-by: Paul Brook <paul@nowt.org>
Merge branch 'usb-serial-begin' of github.com:pbrook/Arduino into pbrook-usb-serial-begin