Fail early when given format for TimeZone

Fixes #201.

`ActiveSupport::TimeZone` doesn't support `.strptime` like `Time` does. It's unlikely that it will ever be added, or that we could add it ourselves. So instead of waiting until run time to blow up, we'll fail right off the bat when you try to build a filter.

@AaronLasseigne can you look this over?