Dramatically improve deskew performance with leptonica

ImageMagick does a pitifully slow job of deskewing, so I looked around for other options. It turns out that Leptonica, the library Tesseract uses image manipulation, has its own deskewing function, and it works great.

On a real benchmark for deskew + OCR on a mixed bag of files, I got 371 seconds as the baseline time, and 216 seconds when Leptonica handled the deskew -- meaning this improvement will reduce the total processing time for a deskew+OCR job by 40%. Looking at deskew without OCR as a standalone test, Leptonica's deskew absolutely runs circles around ImageMagick, getting the job done 30-40x faster. 

It also seems to produce better quality images. I found a few cases where ImageMagick introduces a halftone pattern into the background after rotation, for example.

Leptonica has a PyPI package (pylepthonica) but it's huge and out of date. So I wrote this custom tiny one.

More implementation notes in the commits.

Portability should be fine – it relies on Python's find_library to do the nasty work of finding shared library it needs. Tested on OS X 10.9 and Ubuntu 13.10. I am not sure if it will work on platforms where liblept is compiled as 64-bit and Python is running as 32-bit. I understand if you want to delay this in the interest of stabilizing your v2.0 release.

ImageMagick is still needed (just barely) for an invocation of identify somewhere.