Adding script for freezeing graph (both architecture and weights)

[dboyliao]

Add a simple module freeze_graph.py.
With it, you can serialize your entire vgg graph into a single protobuf file.
It will be much easier to use (you don't have to rebuild the graph again)
Run python freeze_graph.py -h for detail.

[dboyliao]

I use vgg16 as example, but you should be able to do the same thing with vgg19 too.