Add a defaultCredentials so that all sync method will use it if no model credentials are set

Hi,

I'm using this library on a project and it was working perfectly until I called mycollection.create({...}). The collection create the model but eventually doesn't add the credentials to the new model, even if the collection has the credentials. I could have created the model and manually assigned credentials to it before calling the create method, but I didn't like this solution.

Instead, I added a `Backbone.BasicAuth.defaultCredentials` variable. If this is defined and the model or collection doesn't have a credential then the defaultCredentials will be used.