created distance calculation functions and tests.
moved spatial directory out of kdtree folder
copied my ckdtree code into the repo
copied code into brite force pair counters in pairs.py
commented out approx scheme in ckdtree for now.  added testing suite for ckdtree.
moved the __all__ list below imports
updated pairs.pairs to return non-self matches ig given only one list of points.
changed API for pairs.pairs to accept a float for the search radius
removed distance calcs from this file.
Merge pull request #2 from duncandc/develop

Develop
Merge remote-tracking branch 'upstream/develop' into develop.

From Pull request #2, @duncandc. Updates to observables calculation.