All fragment classes now extend the Fragment class

This just adds a `Prismic::Fragments::Fragment` class, so all fragment classes can extend it. It doesn't add any feature, nor does it break anything. It just allows to use Ruby dynamism outside of the kit, in order to expand all of the fragments' abilities at once.
Specifically, I'm thinking about adding a `as_html_safe` method in Rails initializers, so that we don't have to `html_safe` everything prismic.io sends anymore; but that would definitely allow to do more of this kind of things.

Would be cool if we could include it in the gem to come, since it's pretty low-risk.