Allow time_class to be given as an option to Chronic.parse

[lautis]

This implements time_class option to Chronic.parse. You can still set the global default with Chronic.time_class.

The implementation is a bit brute force-ish. I used options hash given to most handler methods to detect time class, but I later realised that the handlers are always included in an object containing the options hash. Options could be refactored away although then the interface would change there a bit.

Fixes #182.