added frequency control of the transmitted wave

A pot on analog pin A0  now controls the note of the transmitted wave by
indexing with its value a constant table filled with absolute
frequencies of musical notes.
usb_mem.h patch

The USB memory management now supports for compile-time variable size
USB packet buffers. MAX_PACKET_BUFFER_SIZE sets the maximum words
available in a buffer, myType sets the words dimension.
added compile-time definition of sampling rate

The 3-byte coded sampling rate is used whenever the Teensy needs a
3-byte coded sampling freq (like during sam_freq_ctrl_request or in
config descriptor)
usb_dev.h patch

This patch uses the 3-byte coded sampling frequency for the
sam_freq_ctrl_request; this patch also sets the size of the Buffer
Descriptor Table entry for any endpoint to the value
MAX_PACKET_BUFFER_SIZE that is to increase the maximum data payload per
transaction.
3-bytes coded sampling frequency in descriptor

The descriptor now uses the defined 3-bytes coded sampling frequency
during enumeration.
#include "usb_desc.h" in usb_audio.c

This include will serve to the audio library to access some useful
constants, without having to redefine everything in every file.