Added admin/util/editable helper for easy inline editing

The helper makes it easy to create editable content on a page without
leaving the page and simply editing it inline with the save function
occuring over AJAX.

It uses the Jeditable jQuery plugin. The plan is to extend it with
support for additional input types and to include the WYSIWYG editor
among those types, but for now the types supported are:

- text
- textarea (which uses the jQuery autogrow plugin)
- select

For more info, see apps/admin/handlers/util/editable.php