enhance examples

@nitishsrivastava,

I slightly enhance examples:
- renamed existing example for MNIST to mnist-ff and changed its input size to more general: 784x1.
- added cnn example for MNIST: mnist-conv (the only new file - net.pbtxt, all others were copied from mnist-ff. Probably at least show_plots.py should be moved somewhere in one place).
- updated README.md with description of updated examples.
- added proposed folder for data: examples/data/
- removed imagenet/CLS_net.pbtxt, as duplicate of CLS_net_20140621074703.pbtxt.