add layer_dict to the python interface

[nitnelave]

Similar to the blobs map, have a layer map.

This turns
net.layers[list(net._layer_names).index("ip2")]
into
net.layer_dict["ip2"]

[ajtulloch]

Sure, seems reasonable to me. Could you add some unit tests?

[nitnelave]

I'd be glad to, but where? :) (Sorry, it's my first time contributing to caffe, and I can't seem to find the tests)

[nitnelave]

Nevermind, I found them in python/caffe/test

[nitnelave]

Here are the tests. To check the equality of the pointed layers, I only checked the types, but since the layers of the test network are all different types anyway, it seemed reasonable.