Change bsdiff's suffix sort algorithm to sais. Fixes #132

Although #132 is marked as closed, the issue is still real. bsdiff can crash or hang when creating diffs on some files. In particular, bsdiff hangs/crashes in its recursive split function.

As you can see from:
http://stackoverflow.com/a/20305493/871119
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=409664

bsdiff 4.3 uses qsufsort for its suffix sorting algorithm, which has a bug causing the hanging/crashing. Our solution is to change the algorithm to use sais for suffix sorting:
https://sites.google.com/site/yuta256/sais

Notes:
*I had to change sais_index_type #define from int to off_t in sais.h/c and in some other places, to eliminate all warnings. Compiles cleanly with clang's -Weverything
*sais is claimed to be more efficient than other algorithms including qsufsort. If a more efficient (and stable) algorithm comes out, we may be able to swap it out in the future.