Check code style on build

Style check the code on compile and fail compilation if there are violations.

I propose using the google java code style guide, which is already pretty close to what we are having now (e.g. does not allow no braces on if/else etc). Spark also uses a modified version of the google code style.

https://google.github.io/styleguide/javaguide.html

https://github.com/apache/spark/blob/master/dev/checkstyle.xml

Supported by many formatters, IDEs, etc like Intellij, clang-format, Eclipse:

https://github.com/google/styleguide