requirejs JavaScript snippets.

Added 2 snippets: for the define and require functions.
Moved RequireJS snippets into javascript.snippets.

The two snippets are at the end of javascript.snippets, and the javascript_requirejs.snippets file has been deleted.
Replaced 'RequireJS' with 'AMD'.

These snippets are more general AMD snippets, rather than being specific
to RequireJS i.e. they can be used by anyone using an AMD script loader,
not just RequireJS.
Merge pull request #95 from ianoxley/requirejs

Javascript: RequireJS snippets.