prevent TypeError when comparing naive and aware dates in get_occurrence()

The date param is potentially timezone-naive (as i ran into with Occurrence.get_absolute_url), so coercing as timezone-aware.  This is perhaps not the best way to have fixed this bug, so please review, but it is a fix nonetheless.
Merge pull request #11 from smcoll/patch-7

prevent TypeError when comparing naive and aware dates in get_occurrence()