EncodeBase64 and DecodeBase64 ops

[vabarbosa]

the TF implementation of the pix2pix model which fails conversion because of

`Unsupported Ops: DecodeJpeg, EncodePng, DecodeBase64`

the Open NSFW model also fails conversion with some of the same ops (tensorflow/tfjs#433).

i would like to take the opportunity to implement some of the ops in TensorFlow.js. starting with this pull request for DecodeBase64 and EncodeBase64.

along with this tfjs-converter PR, there is a corresponding PR in tfjs-core (tensorflow/tfjs-core#1779)

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This change is 

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[vabarbosa]

I signed it!

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[pyu10055]

Thank you for the contribution! approved with couple minor comments.

[pyu10055]

update to 2019 for all files, thanks.

[vabarbosa]

updated (c9ba5f6, fb065c7)

[pyu10055]

this is not needed.

[vabarbosa]

removed (c40b65c)

[pyu10055]

remove the empty line here.

[vabarbosa]

done (7663a37)

[vabarbosa]

@pyu10055 thank you, i have made the updates as requested.

[pyu10055]

This change depends on a PR with tfjs-core, will need to wait for tfjs-core merging that first, and publishing a new version.

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