resolved #461. The collection url should compute from the model instance first.

When you try to create multiple models at the same time, the scope maybe change
depending on the particular record being created, so the URLs should reflect
the scope of the model instance, instead of the global class scope.
Merge branch '461_scope_with_multiple_create' of git://github.com/sishen/spine into sishen-461_scope_with_multiple_create

Conflicts:
	test/specs/ajax.js

Closes #461
Closes #463