examples: cnn_mnist_dag.m: fix CPU-variant

cnn_train.m: fix a bug preventing biases from being learned (closes v…

…lfeat#197))

fix Linux compatibility

cnn_train.m: fixes a bug in computing the errors when using more than…

… one sub batch

Fixed bug reported by Carlos Arteta.

bugfix: CPU compilation (closes vlfeat#72)

vl_compilenn.m: cosmetic fixes

beta6 -> beta7

beta5 -> beta6