Add AES2501 driver

This driver works quite nicely. Seems a little too sensitive though (too
much black in the standardized image, not enough ridge gap definition).

Processing results are quite good, but you need a good enrollment image
(i.e. long!). It's best to get such images by pressing harder than you
might think necessary and swiping slowly.

TODO

@@ -11,7 +11,6 @@ NEW DRIVERS
 ===========
 Sunplus 895 driver
 AES1610 driver
-AES2501 driver
 AES3400/3500 driver
 ID Mouse driver
 Support for 2nd generation MS devices
@@ -19,7 +18,11 @@ Support for 2nd generation UPEK devices
 
 IMAGING
 =======
-aes4000 doesn't work very well, maybe due to small minutia count?
+ignore first frame or two with aes2501
+aes2501: increase threshold "sum" for end-of-image detection
+aes2501 gain calibration
+aes4000 gain calibration
+aes4000 resampling
 PPMM parameter to get_minutiae seems to have no effect
 nbis minutiae should be stored in endian-independent format