Works around the deprication of Protected Attributes in Rails4

* Adds `Zuul.rails3?`, `Zuul.rails4?`, and `Zuul.protected_attributes?` to detect rails version and whether or not the protected_attributes gem is installed.
* Adds `Zuul.should_whitelist?` to roll up the logic determining whether Zuul should inject `attr_accessable` calls.
* Liberally checks `Zuul.should_whitelist?` before injecting said calls.
Merge pull request #3 from jemmons/fix_attr_accessable

Works around the deprication of Protected Attributes in Rails4