Updated ImageNet Tutorial to reflect new merged train+val prototxt format. Also corrected 4,500,000 iterations  -> 450,000 iterations.
Tried to clarify function of `include' lines and train vs. test network differences
Merge pull request #923 from yosinski/doc-update

ImageNet tutorial update to merged train_val.prototext
Merge remote-tracking branch 'upstream/dev' into test