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>'
```
Merge pull request #2 from firmafon/use-inflector-to-constanize

use activesupport inflector to constantize