Add spotless to gradle build for format linting and reformatting

[JLLeitschuh]

This adds the gradle plugin spotless to the build:
https://github.com/diffplug/spotless

With it comes the kotlin formatter ktlint:
https://github.com/shyiko/ktlint

To check if your code is formatted correctly you can run the task spotlessCheck.
To reformat your code to fix any formatting issues, you can run spotlessApply.

The task spotlessCheck is always run as a part of the check task meaning that it is run as part of travis.

It also handles the formatting of the *.gradle.kts files as well.