[MPMD-168] Add parameter to skip report generation if there are no violations/duplications.

 * The report generation for both PMD and CPD is split from the actual
   execution of these tools.
 * PMD and CPD are now executed already when canGenerateReport() is called.
 * The new parameter is called "skipEmptyReport" and is by default "false"
   to remain backwards compatibility.

Please note, that the new parameter is called "skipEmptyReport" and it is by default **not true** to be backwards compatible.
However, thinking over, I guess it's not a big deal to change the behaviour here and it seems to be a better default conventional
configuration.

* https://jira.codehaus.org/browse/MPMD-168
* `mvn clean integration-test -Prun-its` worked fine for me.