Allows for inline template binding of polymer elements

Really simple, just expanded upon what was added for #19. Properly filled out the call to template.createInstance. 

I was using firebase-element's firebase-login element, and wanted to have a global instance of it to pass around to my different page elements. this way my index page is just calling in one polymer element, and it goes from there.

I updated the readme with an example of what I used this for. Ran into this issue on Saturday and found a solution this morning after tracing through the code and trying to do this any other way imaginable... :) 

I would like to allow lazy loading of an element with this variable binding functionality, but this will work great for now.

Thanks for this awesome web component! Happy to make any updates you see necessary.