New backend pieusb

Support for two Reflecta USB scanners:
 * - CrystalScan 7200 (model id 0x30)
 * - ProScan 7200 (model id 0x36)
(OEM is Pacific Image Electronics - PIE)

Backend by Jan Vleeshouwers based on discussions with
Michael Rickmann and Klaus Kaempf
Add pieusb to Makefile.am
pieusb_usb.c: Fix type in call to sanei_usb_read_bulk()