added support for 48 KHz stereo input

The device now supports stereo 48 KHz; a patch to usb_mem.h and
usb_dev..c has been made to enable support for USB buffers greater than
64 bytes; patched usb_desc.h/c to contain info about the number of
channels; patched sketch to do a pair of stereo samples for each
iteratiion; to change bit resolution of the lookup table index, you only
have to change LUT_INDEX_BITS in sine_lut_full.h; NOTE: sampling
frequency, packets sizes and buffers sizes must be coherent in every
file.