fix #170

I want to resolve #170 . Instead of storing a reference to constant I decide to store a constant name string. When we access ```processor_class``` it [constantize]( http://api.rubyonrails.org/classes/ActiveSupport/Inflector.html#method-i-constantize) this string.
How about renaming this to `processor_class_name`, and expecting people to set it to a string instead of a class? That way people can't get this error again. It does mean a breaking change though - @calebthompson, thoughts?