allow plugin to be reinitalized

Hello, 
We are using this plugin in a situation where we needed to swap out the image being zoomed so I slightly modified the original code to accomodate this.  You can now swap out the image url and call the jqzoom function again, <code>$('a.yourClass').jqzoom( zoomOpts );</code> to update the image being zoomed.  Thought this could be useful to somebody else.  

Thanks for putting this plugin together,

Paul