vl_nnrelu.m: cleanup
vl_simplenn.m: significantly optimize memory handling
vl_nnconv.cu: removes spurious message
Merge remote-tracking branch 'origin/master'
cnn_imagenet_setup_data.m: more robust
nnhelper.h: simplifies persistent memory storage using only MATLAB arrays
vl_testder.m: allows specifying a tolerance
vl_test_nnnormalize.m: give more slack to the approximation in the pow function before rising an error
vl_test_nnlayers.m: typo
normalization: dramatic speedup on GPU by coalescing kernel calls
cnn_train.m: fix a bug preventing the momentum-resetting code from working (closes #4)

This patch also accelerate somewhat the gradient accumulation.
Xcode: sync