rangeSeparator option

I came up with a need... When you use the weekView both on agenda and basic mode, I have to customize the text between the dates, for something like dd/MM/YYYY **to** dd/MM/YYYY. When I was analyzing the code, I have found that everything are already implemented, the only issue is that the method t.formatRange on Calendar.js doesn't have the separator parameter to be replied to date-formattig.js formatRange. 

I created an option call rangeSeparator, that is by default set to \u2014, the tests aren't in compliance with the \u2014 character after the change I did... So I have to fix the tests to pass. Everything is working fine now, I did a lot of tests and the PhantomJS is building OK, so, you can automatic merge if you want. Also if you need that I change the docs, to reflect that change, feel free to contact me.

PS: This is the first of a few implementations that I will have to do, since I working on a calendar-based system right now, that is very customized. I am pretty sure that I will face some things that are not implemented yet, such as this rangeSeparator. Gladly, fullcalendar is well coded and documented, so I can understand the code very easily. Maybe from now on, I can contribute with the project :)