Addressing review comments

- changed to a python idiom for default exclusives arguments.
- pruned redundancy from documentation.
- renamed test file.
- changed parameter handling for creation of tasks from the
  TestCheckExclusives test.
- cleaned up exclusives propagation for InternalTarget.

Still todo before this can merge to master:
- Examine all of the target types, to identify the set which are proxies,
  and make sure that all proxy targets propagate the exclusives correctly.

Auditor: benjy