KIJIMR-169. Import Wibi's graph library.

Wibi's graph-modeled machine learning tools (especially pertaining to
item-category association mining) are now available in the package
org.kiji.mapreduce.lib.graph. Further testing may be desirable (new
tickets will be filed as necessary).

Reviewed by: lsheng, natty
Review: http://review.kiji.org/r/280