New method Memcached_DataObject::pivotGet()

New method Memcached_DataObject::pivotGet()

This method lets you get all the objects with a given variable key and
another set of "fixed" keys. A good example is getting all the avatars
for a notice list; the avatar size stays the same, but the IDs change.

Since it's very similar to multiGet(), I refactored that function to
use pivotGet().

And, yes, I realize these are kind of hard to follow.