Added a Backbone-free version

While browsing through the code I noticed that the Backbone-dependency is almost not used at all (only for the `extend` function). So, I thought it might be a good idea to provide a version that does not require Backbone.

This PR probably needs some more work which I'm happy to contribute if you think it is a good idea.

What I did:
* make a new wrapper file `build/syphon.js`
* copied the `extend` function from Backbone source
* changed Gruntfile
* changed tests to use `Syphon` instead of `Backbone.Syphon`