[Core] Slight refactoring in asset generators

 * Added Javadocs
 * A new appendContent(JQueryContent contentToMerge) has been added to JQueryContentGenerator to be able to bufferize Javascript content
 * Renamed some classes
[Core] Added new utilities for Javascript generation

 * JsFunction, easing the generation of a Javascript function using a JSON parser
 * JsSnippet, easing the generation of a Javascript snippet of code
[Core] Moved i18n-related classes from datatables to core

In order to allow other components to use the same resolution mechanism for locale and messages.