Expose model scopes for search criteria

Using `ransackable_scopes`, users can define whitelists for allowed
model scopes.
Fix specs for Rails 3.0 / 3.1

Conflicts:
	spec/support/schema.rb
Fix ruby 1.8.7 issues

* No public_method or public_send in 1.8.7
* No case/when with procs in 1.8.7