Configurable default binding attribute.  Done with the full knowledge that an alternative is to use Backbone.Modelbinder.createDefaultBindings(dom, 'attr') but this feels unwieldy.  Allowing the configuration of the selector seems like a cleaner approach.
Merge branch 'master' of https://github.com/adamterlson/Backbone.ModelBinder

Conflicts:
	Backbone.ModelBinder.js
	spec/javascripts/defaultBindings.spec.js