Linking Frameworks correctly if multiple.

For example python and python3 both provide a
`Python.framework` and OS X Frameworks are made to
deal with this by putting them into `Versions` and
linking the default one as `Current`. However, brew
did not respect this and cleaned out stuff making
neither of the two versions work because `Current`
is replaced by an empty directory.

This commit fixes that. However, still one of the
two `Python.frameworks` has to remove it's
`Current` and `Headers` links, because brew cannot
decide alone which one is the "default" and allowed
to set the `Current` link.

A similar situation might apply to Qt 4 vs. 5..