add missing value support to SoftmaxLossLayer
add spatial normalization option to SoftmaxLossLayer

With missing values (and batches of varying spatial dimension),
normalizing each batch across instances can inappropriately give
different instances different weights, so we give the option of simply
normalizing by the batch size instead.
clean up spacing in SoftmaxLossLayer
move softmax loss GPU todo comment to header file

There's no need to have GPU stubs just for a comment, and the absence of
a GPU-specific implementation is made more obvious by the nonexistence
of the .cu file.
Merge pull request #1639 from longjon/crop-layer

Crop layer for automatically aligning computations
Merge pull request #1313 from shelhamer/reshape-data-layer

Reshape single input batches for inputs of varying dimension
Merge pull request #1654 from longjon/softmax-missing-values

Add missing value support to SoftmaxLossLayer
Merge pull request #1655 from longjon/softmax-loss-todo

Move softmax loss GPU todo comment from .cu file to header file
add creation script
update readme