Enable 'eslint'

[wimrijnders]

This enables linting for vis.js.

• Replaces the babel-eslint (has issues) with gulp-eslint
• Has been added to package.json, so it should be automatically installed.
• Linting has intentionally not been added yet to the default task; there are simply too many errors at the moment to make this comfortable
• Can be run as follows:

 > gulp lint            # Run over all of vis.js
 > gulp lint-graph3d
 > gulp lint-network
 > gulp lint-timeline

The linting has been run over Graph3d, results are added here for reference of what changes this brings.

On the whole, I'm pretty happy with eslint; I hope it comes in regular use.

[wimrijnders]

Removing command-line option -g in .travis.yml seems to fix the travis build problem. I hope this is OK.