Update javaVersion usage in top level pom.xml

The maven compiler plugin used hard coded javaVersion instead of the global property.

The README.md mentions that java 7 is the default version now, but the javaTargetVersion still has the value of 1.6 - Let's update the target version to 1.7 so it is in correlation with the source version. In the meantime mention the overridable javaTargetVersion in the readme too.

With Java 7 there were some new compile warnings introduced which causes the build to fail on modules where the error on warning options is enabled (-Werror), some warnings have to be fixed due to this.