[tests] don't use Gaussian labels in NetTest's TinyNet

Previously, labels were being nonsensically filled with a zero-mean
Gaussian. Now they are just set to constant zero. Note that this is not
necessary for TrickyNet, neither forward nor backward are ever called.
Merge pull request #1661 from longjon/init-test-labels

Fix sketchy label initialization in NetTest
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 formatting in SoftmaxLossLayer
add BaseConvolutionLayer

This provides a common place for code used by ConvolutionLayer and
DeconvolutionLayer, simplifying the implementations of both.
add CPU_ONLY ifdef guards to BaseConvolutionLayer
rewrite ConvolutionLayer to use BaseConvolutionLayer helpers
add DeconvolutionLayer, using BaseConvolutionLayer
zero-init param diffs and accumulate gradients

(With layers whose backwards accumlate gradients), this effectively
decouples the computational batch from the SGD minibatch. Each
iteration accumulates gradients over iter_size batches, then parameters
are updated.
zero-init param diffs in gradient checker
accumulate gradients in inner product layer
accumulate gradients in (de)conv layers
accumulate gradients in cudnn conv layer
add util/coords.hpp for coordinate mapping functions

This will be useful for keeping track of the coordinate transformations
induced by, e.g., convolution and pooling layers.
add FilterMap for the coord mapping used by (de)conv and pooling layers
implement coord_map for all applicable layers
layers get a pointer back to their owning Net

This allows layers to do things that depend, e.g., on net topology.
add CropLayer for cropping one blob to another using induced coordinates
Merge pull request #1639 from longjon/crop-layer

Crop layer for automatically aligning computations