Enhance getPage in PageFactory

New PR to develop instead of master see #89.

Closes #12 
I agree with your proposition at https://github.com/qa-tools/qa-tools/pull/99#issuecomment-62280944 and here we do:

1. rename method to `getExistingPageClass`
2. move exception throwing here (from `resolvePage` method)
3. in the `resolvePage` method just return what `getExistingPageClass` returned with no extra checks