Models imported using ONNX added

[ayush1999]

I've added three models (in this PR):

1. Squeezenet
2. DenseNet121
3. ResNet50

All of them are working fine. Binaries could be added after this PR is merged. (along with the new release maybe)

[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by 25.04%.
The diff coverage is 47.43%.

@@             Coverage Diff             @@
##           master       #6       +/-   ##
===========================================
+ Coverage   11.76%   36.81%   +25.04%     
===========================================
  Files           8       12        +4     
  Lines         221      747      +526     
===========================================
+ Hits           26      275      +249     
- Misses        195      472      +277

Impacted Files	Coverage Δ
src/Metalhead.jl	0% <ø> (ø)	⬆️
src/googlenet.jl	0% <0%> (ø)
src/utils.jl	24.24% <100%> (+7.57%)	⬆️
src/resnet.jl	47.44% <47.44%> (ø)
src/densenet.jl	51.56% <51.56%> (ø)
src/squeezenet.jl	98% <98%> (ø)

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[ayush1999]

@MikeInnes Should I add the binaries now, or after merging this? (Because the Travis CI would fail without adding them)

[MikeInnes]

Go ahead and add the binaries -- let me know if you need access to do so.

For some of these it would be nice to write these out in a more compact way, e.g. that generates all the layers. This is fine for now though.

[ayush1999]

@MikeInnes I might need push access to add the binaries. Also, I was thinking of releasing a new version along with the binaries, if it's not a problem.