Support multiple checkstyle executions

Multiple execution sections may be defined in pom.xml e.g. in order to provide different rules for unit tests and production code.
- Plugin Wrapper contains one MOJO execution for each execution found in checkstyle executions section in pom.xml. The generated Eclipse checkstyle config contains a file set and config for each one.
- The source and test source folder of the Maven checkstyle configuration are evaluated. Previously, they were always set to the default values.
- Findbugs and PMD throw an error if more than one MOJO execution is found.