Modernize project

[stephenmathieson]

This patch "modernizes" the axe-webdriverjs project. I've found it's quite difficult to contribute here, as the project used outdated tech and had confusing lint rules setup.

To help make this an easier project to work on, I've done the following:

• replace Grunt with npm scripts
• replace JSHint with ESLint
• add Prettier for code formatting
• made the integration tests more self-contained (they no longer require you to start a static file server on your own)
• fix code coverage generation (by using nyc)
• updated CircleCI to use the v2 configuration (v1 will be sunset soon)

See individual commits for a much easier review.