settings change; more unit testing; bug fix

settings.py now supports inclusion of another file, local_settings.py,
for storing local overrides.

added several unit tests for testsuite import and upgrading; unit
testing now works if you disable django_evolution.

bug fix: don't clear flagged_for_review in evaluation.save() but rather
in the form post for edit evaluation. this means if the flag was set,
clear it after the user edited the evaluation.

bug fix: allow tests with duplicate IDs (note: this is not the internal
database ID but rather the testsuite identifier). this will happen
every time we upgrade the test suite.

bug fix: divide by 0 error in edge case for scoring