jquery.documentReady.js : fix when $ is used multiple times

Hi,

I fixed jquery.documentReady.js to allow the $ function to be called multiple times.

The following jsfiddle may speak more clearly than a long description:

 * Without fix: http://jsfiddle.net/p8nxt/
 * With fix: http://jsfiddle.net/8j85t/ -> it seems to work really like the original jQuery

My fork address the issue already told by vinin3 here: https://github.com/addyosmani/jquery.parts/pull/12

However my changes are coded in a different way.

To answer some questions that may come (cf previous link):

 * My changes don't come from jQuery.
 * However the idea and the implementation are rather simple, and my changes seems very unlikely to prevent updates coming from jQuery core. (I admit I have not done all checks for this point yet, I can have a further look if it may help.) 

Hoping my fix may help,

Regards,

David Dallet