Add tests for credit loading and tag pool creation forms.
Set alert_threshold to a Decimal in test_load_credits (Python 2.6 doesn't allow converting a float directly to a Decimal).
Fix asserts to use Decimals.
Fix bug in load_credits under Python 2.6 (it tries to coerce float to Decimal).
Merge branch 'feature/issue-826-add-tests-for-billing-forms' into develop