descriptor changed for both in/out

The Teensy 3.0 now enumerates as a full duplex stereo device.
device stack supports requests for OUT interface

- Added some callback functions for the OUT interface to let the device
stack enable or disable the corresponding AudioStreaming interface
- USB receive functions now support packets with size > 64 byte too.
corrected MAX_PACKET_BUFFER_SIZE define

This costant value was bigger than necessary.
usb_malloc() modded

This function now counts how many myType slots are left in memory,
rather than how many uint8_t.
Added functions to actually receive data
New sketch for full duplex

This sketch gets data from USB, sends it back to USB.