Reap non-persistent arrays atexit, and after context managers.

Remove attempt at tying __del__ to database deletion, since there
appears to be no reliable way to attach events to garbage collection.

Fixes #16
Raise exception when reaping persistent arrays
bugfix
Merge pull request #23 from ChrisBeaumont/gc

Reap non-persistent arrays atexit, and after context managers.
Refactor tests to use pytest.

Fixes to make sure suite runs on python 2.6 without pandas or scipy
Fix merge conflict