Add default prefix to the Maven execution IDs in jacoco examples.

As discussed in https://groups.google.com/d/msg/jacoco/NuJc2YgTsSk/X9ccG4iL7oYJ.
Further information on Maven execution IDs:
* http://maven.apache.org/guides/mini/guide-default-execution-ids.html
* http://docs.codehaus.org/display/MAVENUSER/Default+Plugin+Execution+IDs

On my machine a run of `mvn clean verify` did succeed.