Add a MaxPool object

[dpsanders]

We should add a MaxPool object, so that displaying the layers in eg. VGG19 looks better:

julia> vgg.layers.layers
28-element Array{Any,1}:
 Conv((3, 3), 3=>64, NNlib.relu)   
 Conv((3, 3), 64=>64, NNlib.relu)  
 Metalhead.#52                     
 Conv((3, 3), 64=>128, NNlib.relu) 
 Conv((3, 3), 128=>128, NNlib.relu)
 Metalhead.#53                     
 Conv((3, 3), 128=>256, NNlib.relu)
 Conv((3, 3), 256=>256, NNlib.relu)
 Conv((3, 3), 256=>256, NNlib.relu)
 Conv((3, 3), 256=>256, NNlib.relu)
 Metalhead.#54                     
 Conv((3, 3), 256=>512, NNlib.relu)
 Conv((3, 3), 512=>512, NNlib.relu)
 ⋮                                 
 Conv((3, 3), 512=>512, NNlib.relu)
 Conv((3, 3), 512=>512, NNlib.relu)
 Conv((3, 3), 512=>512, NNlib.relu)
 Conv((3, 3), 512=>512, NNlib.relu)
 Metalhead.#56                     
 Metalhead.#57                     
 Dense(25088, 4096, NNlib.relu)    
 Flux.Dropout{Float32}(0.5, false) 
 Dense(4096, 4096, NNlib.relu)     
 Flux.Dropout{Float32}(0.5, false) 
 Dense(4096, 1000)                 
 NNlib.softmax   

[MikeInnes]

Would be happy to see a Flux PR for this (should only be a couple lines).

[MikeInnes]

This will be added to Flux by FluxML/Flux.jl#339, so once that's in we can use it here.

[yuehhua]

This has already been added by #43, so this issue can be closed.