Implement ctypes wrapper around Leptonica to access its deskew function

A few design notes:
Leptonica's deskew is far superior to ImageMagick's convert -deskew command --
around 30-40x faster.  Subjectively the output appears to this contributor to
be of higher quality as well.  The difference is the algorithm: ImageMagick
uses the complex Hough transform to find the skew angle, while Leptonica uses
the simpler method, Postl's variance of differential line sums -- conceptually, shear the image and check for straight horizontal.  In this case
simplicity wins.  Finding the skew angle is the bulk of the work.

Leptonica's author explains the advantages of his approach here:
http://www.leptonica.com/skew-measurement.html

Leptonica is the low-level library that Tesseract depends on.  Hence, this
project already depends on Leptonica.  Leptonica can read and write most
common image file types on its own.

Unfortunately its error handling is poor: it seldom returns any meaningful
error codes.  The best it manages is writing messages to stderr, which in
the context of a verbose script is just confusing since the error's source
is not indicated.  The problem is compounded by Tesseract's use of Leptonica,
which will produce exactly the same errors in some cases.  So we trap stderr
between calls to Leptonica and parse it for a few different types of error
message.

leptonica.py is Python 2/3 compatible and set up to provide access to other
Leptonica functions as needed.  Of particular interest are its orientation
detection (including flip and rotation errors) which it does by comparing
text ascenders to descenders.

There is a PyPI "pylepthonica" package, however it is out of date by a few
years, and it implements all of Leptonica with Python wrappers -- so it is
massive, with one .py file at 2.5 MB.  This module is loosely inspired by
pyleptonica but more modern, up to date, and contains only limited
functionality.
Replace ImageMagick-convert with Leptonica
Merge branch 'for-upstream/leptdeskew' into develop