Apply license gradle plugin

[rdsr]

We should add the license gradle plugin - https://github.com/hierynomus/license-gradle-plugin , which would automatically check that all files have license headers

[mccheah]

We propose this as a subset of gradle-baseline in #28. I don't see why we would need a separate license Gradle plugin if we can satisfy the same requirements using Checkstyle as Baseline does - thoughts?

[rdsr]

@mccheah makes sense. I should have looked into the previous ticket more thoroughly. I'll close this