feature #263 Updated Encore object to class for proxy typehinting (florentdestremau)

This PR was squashed before being merged into the master branch (closes #263).

Discussion
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Updated Encore object to class for proxy typehinting

Hello, I'm trying to address #151 with this PR.

I have an error in my tests:

```
Public API
    setOutputPath
  Error: Encore.configureRuntimeEnvironment is not a recognized property or method.

  - index.js:942 Object.get
    /home/florent/dev/opensource/webpack-encore/index.js:942:27
  - index.js:17 Context.beforeEach
    /home/florent/dev/opensource/webpack-encore/test/index.js:17:13
  - runnable.js:348 callFn
    [webpack-encore]/[mocha]/lib/runnable.js:348:21
  - runnable.js:340 Hook.Runnable.run
    [webpack-encore]/[mocha]/lib/runnable.js:340:7
  - runner.js:309 next
    [webpack-encore]/[mocha]/lib/runner.js:309:10
  - runner.js:339 Immediate.<anonymous>
    [webpack-encore]/[mocha]/lib/runner.js:339:5
  - timers.js:789 runCallback
```

I'm looking into it but any advice would be welcome so that we all get a proper proxy and autocomplete on :heart: *PHPSTORM* :heart:

Commits
-------

fb7498d Initiated class in proxy, uniformized @returns
79dbee8 Updated Encore object to class for proxy typehinting

Author: weaverryan