Improved documentation ...

Class member documentation improved via removal of unnecessary @memberofs - more members now show up in the docs.
Class getter/setter documentation improved using technique above, and also by removing documentation for setters. Documentation was only showing the setter docs, whereas the getter is where the important information is. JSDoc maintainers themselves recommend just doccing the getter jsdoc3/jsdoc#973
Documented a few classes to let devs know that they are automatically created by PIXI and added to renderer.plugins, like interactionManager and prepare

Renamed uploadHookSource to uploadHookHelper.

Combined Prepare shared code, added limit variant.

Huge refactor to match a new shiny eslint file.

CanvasPrepare updated to simulate WebGLPrepare (#2959) ...

* For WebGLPrepare- the done callback is now optional
For CanvasPrepare - an implimentation has been added rathar than it just being a stub. Uploading textures to the GPU can be simulated by forcing a draw command onto a hidden using these textures.

* Fixed couple of context issues

Update imports and exports, getters and setters

Disallow the use of `var`

Implementing classes using the new ES6 syntax (#2936) ...

* Migration to ES6 classes

* Migration to ES6 classes

* Migration to ES6 classes

* Migration to ES6 classes

* Using babelify to transpile ES6 code

* Adding babel-preset-es2015 in devDependencies

Docs update (#2651) ...

* Epic run through the docs

* Fixed merge issue

* PIXI.utils now outputs its documentation correctly

Adds missing destroy to Prepare, supports multiple simultaneous uploads

Adds ability to register additional upload hooks, adds canvas prepare