major update

-Fixed error at line 484 in usb_desc.c, the bSubFrameSize of the Type I
Format Descriptor has been corrected to 2 bytes per sample
-Deleted BlinkAndTalk.ino sketch: it has been replaced by two new
example sketches, one to produce a squarewave, and one to produce a
sinewave.
-Configured the device to Synchronous mode operation: the sample
production is now slaved to the USB Start Of Frame timer.