Add support for react native environment

[bnjm]

Scope & Features

• Adds support for running 3dio-js from React Native by providing a work around for the following metro-bundler issue: Getting "require() must have a single string literal argument" when bundling moment.js facebook/metro#65

Design

• Dynamic require calls can be made from node and react native environments as io3d.runtime.require('../module')
• Runtime now checks for React Native

How to use

• Dynamically require a module:

if (runtime.isNode) {
  return runtime.require('node-fetch')
}

• check for React Native

io3d.runtime.isReactNative 
// -> boolean

• These changes do not effect how existing consumers interact with 3dio-js

How to test

• Tests which require a mock runtime, for example https://github.com/bnjm/3dio-js/blob/4a5b2ae64b6efba6a8a47535ad72cde52646dea9/test/scene/structure/get.js,
  have to also include the mock require like shown in the file

Notes

• It is possible that future versions of metro-bundler might support dynamic requires, but this should be seen as a stable workaround. This was also the approach adopted by the moment.js community to work around this issue Allow compilation without dynamic requires moment/moment#4187

Edit: There are few unnecessary commits here as this was built off an earlier experiment in my fork, happy to squash these where possible.

[bnjm]

Attached issue: #89

[tomas-polach]

looks good! good to go from my side