Change descriptor and code to handle features for USB Audio

https://forum.pjrc.com/threads/33017-Teensy-3-1-Mouse-horizontal-scroll

This patch simply checks if NUM_ENDPOINTS is defined, and will bypass the
built-in USB descriptor, device and memory code, and allows a custom setting.
This bug does not affect the IDE unless someone adds the custom option to
the boards.txt file, and provides replacement code in a library or sketch.
It can also be used to turn off USB by providing no replacements, while
allowing speeds >= 20MHz.

http://forum.pjrc.com/threads/25482-Duplicate-usb-modem-number-HELP!