Make build fail on compiler warnings

[benedeki]

As a sign of maturity, it would be nice to ensure warnign don't (re-)appear during compliaton of the code. Change the project settings to fail in that case.

Depends on #91

Suggested solution:

Add the following configuration into the pom.xml files

            <plugin>
                <groupId>net.alchim31.maven</groupId>
                <artifactId>scala-maven-plugin</artifactId>
                <version>${maven.scala.version}</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                            <goal>testCompile</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <args>
                        <arg>-Xfatal-warnings</arg>
                        <arg>-unchecked</arg>
                        <arg>-deprecation</arg>
                        <arg>-feature</arg>
                    </args>
                </configuration>
            </plugin>

[dk1844]

It is possible that

<arg>-target:jvm-1.8</arg>

might be needed for the s3-sdk code, too.

[benedeki]

It is possible that

<arg>-target:jvm-1.8</arg>

might be needed for the s3-sdk code, too.

I think this is already there. (So yeah, should stay)