Enforce coding conventions at compilation time

[Lior Zeno]

Hi guys, we should make the reviewing process easier and more focused on
correctness rather than style issues. I suggest enforcing our code style (
https://cwiki.apache.org/confluence/display/FLUME/Code+Formatting) at
compile time using the maven checkstyle plugin (
https://maven.apache.org/plugins/maven-checkstyle-plugin/). This will make
the reviewing process easier, and will make sure that all committed code is
strictly following our code style.
What do you think?
Thanks