switch for use in plugin environments.

renamed the objects and generally improved documentation.

Also added an operator<< for dnn_trainer that prints the parameters it's using.

These changes also break backwards compatibility with the previous
serialization format for dnn_trainer objects.

checks if the user is giving truth boxes that can't be detected because the
non-max suppression settings would prevent them from being output at the same
time.  If this happens then we print a warning message and set one of the
offending boxes to "ignore".

code from java.

libpng than they built against.