Update license headers and add validation to 'check'

[tolbertam]

Ahead of the incoming PR for #76 , I thought it would be good to add license-gradle-plugin for adding, updating and validating license header presence in all source files.

License validation will be done automatically as part of the check task. The build will fail if the header is missing from any file.

To update all files with the LICENSE header, simply run:

./gradlew licenseFormat

See b4d046a for the gradle changes specifically.

[tolbertam]

downgraded to 0.11.0 (from 0.13.1) of the plugin since the latest version requires JDK 7 (hierynomus/license-gradle-plugin#35). At some point we could consider removing JDK6 build support and use something like animal-sniffer to ensure JDK6 support is retained..can look into that if you think that is a good route.

[chbatey]

Thanks @tolbertam