Move tfjs dependency; fix with-node example warnings

[vvmnnnkv]

This should resolve #97:

1 (with-node example warnings problem):

• with-grid example is fixed to not include tfjs into bundle because it's already included in index.html (this solves double init warnings).
  Additionally:
• tfjs dependency is moved to dev dependencies section. We still need it for executing tests, but we don't want syft.js installation to pull tfjs.
• tfjs is listed as peer dependency, which means syft.js expects tfjs to be already installed.
• rollup config uses rollup-plugin-peer-deps-external that automatically excludes peerDependencies from syft.js bundle.
• tfjs dependency is updated to lighter variant tfjs-code. Currently we don't need layer API, so we don't have to require it to be installed.

2 (warnings about tfjs-node during unit tests):
One solution to that could be including tfjs-node in dev dependencies, but I found this involves TF C++ bindings compilation step when you do npm install, which can be even more annoying than warnings during tests.

Note:
I checked tfjs model packages, and it seems we do similar.
E.g. a library with the model https://github.com/tensorflow/tfjs-models/tree/master/posenet using rollup and also having tfjs as peer dependency.

[codecov]

Codecov Report

Merging #98 into master will increase coverage by 14.81%.
The diff coverage is 94.73%.

@@             Coverage Diff             @@
##           master      #98       +/-   ##
===========================================
+ Coverage   71.15%   85.97%   +14.81%     
===========================================
  Files          21       18        -3     
  Lines         676      556      -120     
  Branches       85       80        -5     
===========================================
- Hits          481      478        -3     
+ Misses        174       67      -107     
+ Partials       21       11       -10

Impacted Files	Coverage Δ
src/types/pointer-tensor.js	100% <ø> (+33.33%)	⬆️
src/types/message.js	84.31% <ø> (+18.07%)	⬆️
src/syft.js	47.82% <ø> (ø)	⬆️
src/protobuf/index.js	92% <100%> (ø)	⬆️
src/types/torch.js	90.9% <100%> (+30.19%)	⬆️
src/types/placeholder.js	100% <100%> (+60%)	⬆️
src/types/protocol.js	100% <100%> (+23.07%)	⬆️
src/types/plan.js	95.45% <80%> (+25.08%)	⬆️
src/webrtc.js	90.66% <0%> (+5.33%)	⬆️
... and 3 more

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