Prior to this change, different function objects declared their return type in different ways,
now this has all been reconciled.  Now they all declare it as a public typedef named result_type.

I also simplified the cross_validate_multiclass_trainer(), cross_validate_trainer(),
test_binary_decision_function(), and test_multiclass_decision_function().  They now always
return double matrices regardless of any other consideration.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404175