Fixed Model's url generation if urlRoot is a function

Backbone 0.9.0 introduced a change to allow Model#urlRoot to be a
function or value – this fixes Model#url to work with both types.
Fixed extra GET on creation with empty response.

Backbone 0.9.9 and newer are executing callbacks with its own
arguments rather than these passed by jQuery.ajax callbacks.
Tastypie SessionAuthentication support
Add CSRF into headers