unit tests.

of TCP connections in the TIME_WAIT state on Windows XP.  This results in the tests failing
if you run them multiple times within the TIME_WAIT timeout period.  So this change makes
the tests less likely to false alarm when run on Windows XP.

matrix with a sparse vector.

like normal rather than passing it back by reference.

unit test code.

it interprets its epsilon parameter the same way the other OCA based SVM tools
in dlib do.  It also has a more intuitive interpretation than the previous
stopping condition.

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.