Fix issues with karma hanging on exit

[bjacobel]

See issue here. karma-coverage with reporters text and text-summary and our setup for some reason has an issue where it will hang for ~30 seconds prior to exiting. Since coverage reports are only marginally helpful in development, this PR removes the text-summary reporter. (It doesn't affect Coveralls.)

Also tweaks a few other things around the Karma setup:

• adds ability to run tests with npm test, which means you don't need to install Gulp globally
• removes a library that wasn't doing anything
• fixes linting issues

cc @andy-armstrong

[andy-armstrong]

👍 Thanks for tracking this down as it had been bothering me too.

[coveralls]

Coverage remained the same at 92.874% when pulling 3258744 on fix-karma-hang into 9759688 on master.