Implemented Padrino hook in a more Padrino way

Hi guys,

First of all, thanks a million for having a Padrino example in place :). Much appreciated!! I was reviewing our plugin requests and spotted one for `secureheaders` padrino/padrino-recipes#46 (ref padrino/padrino-framework#1048), so I came around your gem and found that we could make the integration a bit more Padrino style.
This PR implements a register hook closer to the way we load most gems. It also hints that the configuration should be moved towards `before_load` in `config/boot.rb`.

What do you think about that?
Thanks, :)
Darío

/cc @postmodern @achiu