replace ForeachTask::createFileSet() with addFileSet()
Use parent class add*() method if the task has no creator/adder
for the class itself.

This allows using PearPackageFileSet with all tasks that have a
addFileSet() method.
replace createFileSet() with addFileSet() wherever possible
adopt pearpackagefileset documentation example to new syntax
Merge pull request #275 from cweiske/parenttype

Use parent class add*() method if the task has no creator/adder for the class itself.