Add intersection() + rename merge() to union()

Twix.js already implements merge(). Merge is in fact the [union](http://en.wikipedia.org/wiki/Union_%28set_theory%29) between two ranges.
Next logical step is to add intersection().
intersection() computes the [intersection](http://en.wikipedia.org/wiki/Intersection_%28set_theory%29) between two ranges.

Why having intersection() is interesting? in order to achieve this:

![Google Calendar](http://img11.hostingpics.net/pics/924040ScreenShot20130427at101217PM.png)