Revert "Merge pull request #320 from bricker/fix_nil_math_errors"

This reverts commit d63ac017a9d115879e3c68684911cc7dea86c025, reversing
changes made to f77fae765ac75ce5ac95f47a7b8d70d0653ecf26.

Conflicts:

	lib/kaminari/models/page_scope_methods.rb
	spec/models/active_record/scopes_spec.rb
Revert "Merge pull request #304 from bricker88/nil_for_per"

This reverts commit 38fb04fab3e6c094a2325ce5e9d9d34051095c07, reversing
changes made to 992fb9f935ca6290d4abf931e935cab4e1fd5b5a.

Conflicts:

	README.rdoc
	spec/models/active_record/scopes_spec.rb
Use default_per_page value when per(nil)

closes #356