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 (screenshot from Google Calendar):

![Google Calendar](http://img11.hostingpics.net/pics/924040ScreenShot20130427at101217PM.png)

Example:
The intersection between "Mon Apr 29" (`new Twix("2013-04-29", "2013-04-29", true)`) and the event "Switch End-to-end [...] review" (`new Twix("2013-04-29 14:00", "2013-04-29 15:00", false)`) is "2:00pm - 3:00pm".