Add current-week class

[alexwgraves]

This adds a current-week class to the tr element that contains today's date, to assist with styling for the month's current week. It should behave largely the same as the td_classes_for(day) method – it adds week to each tr and current-week only to the current week of the month.

I've added a couple tests for this to calendar_spec.rb as well.

[excid3]

Great little addition! Thanks @alexwgraves

[rodrigoargumedo]

This is pretty great add! I might start using this in my later projects.