[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
In the first Iteration i used reflection to check for existance. Now it could really be removed as long as wie don't do any additional checks and rename it to `isClassValid`. Mocking it would also make unit tests easier.