use activesupport inflector to constantize

in ruby 1.9, `Object.const_get` fails on namespaced strings, e.g.

```ruby
class Foo
 class Bar
 end end

Object.const_get "Foo::Bar"

=> >> Object.const_get "Foo::Bar" NameError: wrong constant name Foo::Bar
       from (irb):5:in `const_get'
       from (irb):5
       from /usr/local/bin/irb:12:in `<main>'
```