Fixed improper inclusion of activesupport

This gem is broken! As the gem relies on `String#camelize`, a core extension in `activesupport`, there are two necessary changes:

1. The dependency for `activesupport` must be declared in the gemspec, as a ‘runtime’ dependency.  
2. The library incorrectly loads the `activesupport` gem.

    I’m not sure about past Rails versions, but as you can read [the official docs](http://guides.rubyonrails.org/v3.2.13/active_support_core_extensions.html), usage of activesupport requires loading a specific component or group of components. The gem only requires `String` extensions, so requiring `active_support/core_ext/string` is ideal.

I can’t see how it would be necessary, but I can create an issue with a full bug report if desired.

Note: I can see how these issues would be easily overlooked (most people can develop gems that load activesupport without having it in the gemspec, because most people already have activesupport installed!). I like this gem and would be interested in maintaining it if possible. For instance, I’m creating my own fork to add a `to_hash` method and possibly more refinements later on.