automated publication of documentation on parquet.io

- run "mvn site-deploy" to publish javadoc and coverage report to http://parquet.io/parquet-mr/
- feel free to update index.html: https://github.com/Parquet/parquet-mr/blob/gh-pages/index.html
- you need to configure your authentication in ~/.m2/settings.xml to be able to run it.
See: https://github.com/github/maven-plugins