WIP: Add TreeFinder class based on cKDTree.
WIP: link_iter uses can use KDTree (generalized by Dan to maintain Hash too)
TST: Move old trackpy tests into mr's unit test group.
TST: Run tests on all combinations of link/neighbor strategies.

WIP: Fixed typo. 2 Errors, 1 Failure remain.

TST: Make test files executable.
ENH: Finish making KDTree candidates compatible with Tom's subnet functions.
API/TST: Add neighbor_strategy and link_strategy options to link().
Merge pull request #17 from danielballan/ckdtree

Incorporate KDTree code.