More keyphrases for simple string testing
Implement linear suffix array costruction

* Naive suffix-comparing based approach to suffix array construction has been
  replaced here by a linear algorithm [Kärkkäinen & Sanders, 2003], with its
  implementation taken from te pysuffix package (https://code.google.com/p/pysuffix/),
  but partially rewritten with NumPy arrays.
* Removed unused code.
* Fixed a bug in the matching score calculations in suffix array-based AST implementation.