check that jquery is loaded before using

solves #156 #211 #215 indirectly

Instead of blindly calling jQuery in `deform.js`, first test that it's loaded before calling.  This way you can now postpone loading jQuery until the end of a document and then make your own call to `$(document).ready(function(){ deform.load(); });`.  Without this change, the forced call to jQuery simply throws an error and no further javascript is processed.