Fix lint warnings

[jperedadnr]

Adding to the maven compiler plugin:

                   <configuration>
                        <useIncrementalCompilation>false</useIncrementalCompilation>
                        <showWarnings>true</showWarnings>
                        <compilerArgs>
                            <arg>-Xlint:all</arg>
                            <arg>-Werror</arg>
                        </compilerArgs>
                    </configuration>

will show a number of warnings that we are simply ignoring.

It would be good if most of those could be fixed.