Convert paths into Mapping objects

[guiwoda]

This PR adds a function that can convert an array of paths into an array of Mapping objects by reflecting on declared classes, just as the AnnotationDriver does.

The FluentDriver API is almost unchanged. To support receiving already-instanced Mapping objects, the addMapping methods had to be relaxed. Took the opportunity to refactor that relaxation to the lowest-level method, but this could be rolled back if BC matters.

We are debating between this and #48 :smile:

[eigan]

@guiwoda Any reason why this wasn't merged?