Added factory support for node.js

Adds a create() method to the Handlebars object which will generate a new instance of Handlebars in node.js.  The npm test is passing.  I am not sure how the browser version is tested.  Please let me know if there are any issues or feedback.  

I really love the interfaces and DI hooks in handlebars.  We are moving to it for our main rendering engine.  Our company hosts a couple hundred sites and the singleton pattern is blocking our migration.  Ideally, I would love to have this merged to core and published to npm sooner rather than later.  If this is not possible, then we'll run from our fork in the near term.

I am here to help with anything.  Please le me know if there is anything I can do to help.