Added an excludes attribute that is set to an empty set for all SourcePayload subclasses

Ran into this exception:

$ ./pants goal idea src/thrift::

Exception caught:
  File "/Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py", line 201, in <module>
    main()
  File "/Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py", line 196, in main
    _run()
  File "/Users/zundel/Src/pants/src/python/pants/bin/pants_exe.py", line 177, in _run
    result = command.run(lock)
  File "/Users/zundel/Src/pants/src/python/pants/commands/goal.py", line 298, in run
    return engine.execute(context, self.phases)
  File "/Users/zundel/Src/pants/src/python/pants/engine/engine.py", line 50, in execute
    self.attempt(context, phases)
  File "/Users/zundel/Src/pants/src/python/pants/engine/round_engine.py", line 186, in attempt
    phase_executor.attempt(explain)
  File "/Users/zundel/Src/pants/src/python/pants/engine/round_engine.py", line 42, in attempt
    task.execute()
  File "/Users/zundel/Src/pants/src/python/pants/backend/codegen/tasks/code_gen.py", line 130, in execute
    dependees_by_gentarget.get(target, [])
  File "/Users/zundel/Src/pants/src/python/pants/backend/codegen/tasks/apache_thrift_gen.py", line 185, in createtarget
    return self._create_java_target(gentarget, dependees)
  File "/Users/zundel/Src/pants/src/python/pants/backend/codegen/tasks/apache_thrift_gen.py", line 203, in _create_java_target
    return self._inject_target(target, dependees, self.gen_java, 'java', create_target)
  File "/Users/zundel/Src/pants/src/python/pants/backend/codegen/tasks/apache_thrift_gen.py", line 225, in _inject_target
    tgt = create_target(files, deps)
  File "/Users/zundel/Src/pants/src/python/pants/backend/codegen/tasks/apache_thrift_gen.py", line 202, in create_target
    excludes=target.payload.excludes)

Exception message: 'PythonPayload' object has no attribute 'excludes'