Re-add gcal.js file + import FullCalendar source code

Hi,

Following our discussion here : https://github.com/bokmann/fullcalendar-rails/commit/0b1b3506ce81cab733fb840749852eafc626e593#commitcomment-1416352

This re-adds gcal.js and namespaces FullCalendar files.

You will see that the implementation differs from yours.
Instead of using the files generated by FullCalendar build system, it directly integrates FullCalendar source code and uses asset pipeline include feature to easily recreate fullcalendar.js, gcal.js, fullcalendar.css and fullcalendar.print.css.

Why is that? to add more flexibility.

### Why is flexibility needed?

If FullCalendar was well maintained there would be no issue. It is not the case and many forks arose.
On FullCalendar pull requests page https://github.com/arshaw/fullcalendar/pulls there are 31 open requests!

By directly including FullCalendar source code you allow users to easily use one of the many forks without having to rebuild the fork.

This simplifies the traditional and time consuming steps:
change source code/use a fork -> rebuild -> import files into gem -> use within final web app
to:
change source code/use a fork -> import files into gem -> use within final web app

plus FullCalendar build system depends on Java and make, so you have to install both.

### Issues

Rails asset pipeline does not provide a feature to replace `@VERSION` inside intro.js and main.css (or at least I didn't find a simple way to do it - a rake task might do it however).


Regards,