concat layer's backward() method.  It was assigning the gradient to previous
layers instead of adding the gradient, as required by the layer interface
specification.  This change also noticeably speeds up concat layers since only
one CUDA kernel launch now happens per concat operation, rather than one
kernel launch for each sample in a tensor.

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".