ranking_pair.  Also improved wording of comment in svm_rank_trainer_abstract.h

sparse_linear_kernel.

since it might be expensive to call depending on how the user implements it.

user friendly.

serialization.

--HG--
rename : dlib/timer/timer_kernel_2.cpp => dlib/timer/timer.cpp
rename : dlib/timer/timer_kernel_2.h => dlib/timer/timer.h
rename : dlib/timer/timer_kernel_abstract.h => dlib/timer/timer_abstract.h
rename : dlib/timer/timer_kernel_1.h => dlib/timer/timer_heavy.h

These are the timer and timer_heavy objects now.

right away.