Requirejs native support

RequireJs is now natively supported thanks to the global template variable "require".
Assigning a value to this variable push thi library on the stack that will be imported on page load.

A libraries.json file has been added in libraries folder and will contain the RequireJS configuration.

To load a theme or app library, just use the following templates to load these: 
* ```"apps!app_name"``` to load the main.js file located in app_name/js/
* ```"apps!app_name/file1"``` to load the file1.js file located in the same place
* ```"themes!theme_name"``` to load the main.js file located in themes/theme_name/js/

Future improvements: 
* WityCMS library to automatically load libraries on Ajax call
* Easier to load libraries from a theme