Merge pull request #19 from danielballan/api-fix

BUG: Remove sql.py from API.
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: Make KDTree candidates compatible with both of Tom's subnet functions.