3.0 - fix plugin associations

While investigating cakephp/app#17 and updating the documentation, I found a few thorny points in the ORM regarding plugins. The first problem was that using `Photos.Photos` as an association name, resulted in a similarly named association. This association became impossible to `contain()` as `.` has specific meaning in contain().  The second issue was even once those relations were loaded the hydrated property also contained a `.`. This was totally un-expected for me, so I corrected the behavior to better match how the ORM worked in 2.x