Imported libdivsufsort into the project tree

This eliminates the need to install livdivsufsort separately, and allows us to
make changes that would benefit our application.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Use linear time LCP array construction algorithm due to Kasai et al.[1]

Thanks to Timo Duchrow for pointing me to M. Douglas McIlroy's
implementation[2] of this algorithm, which was used as an inspiration for this
implementation.

1. "Linear-Time Longest-Common-Prefix Computation in Suffix Arrays and Its
   Applications", Toru Kasai, Gunho Lee, Hiroki Arimura, Setsou Arikawa and
   Kunsoo Park.

2. http://www.cs.dartmouth.edu/~doug/sarray/ [2013-03-07]

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>