[WIP] Enhance getPage in PageFactory

This is the first WIP commit, just a draft.
* Only the prefix logic is implemented and I am not sure  if the DefaultPageLocator should return an FQCN or a `Page` instance (in this case it requires a `PageFactory`).

Closes #12
I'm not sure if we need that level of abstraction. Why not just use `class_exists` method directly, where needed.