ADD ability to load only modules in given nodes

There is no method allowing to load modules defined only in a given part
of the DOM. This means if nodes are added through ajax, there is no esay
way to load modules for this brand new node.

Related #8

Details
* ADD method `Darwin.Loader.load_modules( $element )`
* UPDATE doc
Merge pull request #9 from MaximeD/ability_to_load_parts

woops, thought I did just that.