This library is cleaned up and minfied version
of code represented at
http://vedder.se/2012/07/usb-serial-on-stm32f4/

Useful resources:
http://www.beyondlogic.org/usbnutshell/usb1.shtml
http://www.usbmadesimple.co.uk/index.html
http://www.linusakesson.net/programming/tty/
