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.
I don't think we get much benefit from that. We're already calling `to_s` which will be a no-op on string values, and this is slightly more flexible. I'm not convinced we benefit from the api change.