Error Prone Static Analysis Tool

[cass-green]

Looks like you're not using any error-checking in your Java build. This pull requests adds a static analysis tool, Error Prone, created by Google to find common errors in Java code. For example, running mvn compile on the following code:

public boolean validate(String s) {
	return s == this.username;
}

would identify this error:

[ERROR] src/main/java/HelloWorld.java:[17,17] error: [StringEquality] String comparison using reference equality instead of value equality
[ERROR]     (see https://errorprone.info/bugpattern/StringEquality)

If you think you might want to try out this plugin, you can just merge this pull request. Please feel free to add any comments below explaining why you did or did not find this recommendation useful.