appledoc 2.1

* Add Appledoc 2.1 to continue to support projects with undocumented
  enumerated and bitmask types.

* This is identical to the last version of homebrew/appledoc that
  referenced 2.1 (mxcl/homebrew@33aa810) except for the comments at top
  and the replacement of `def test` with a block-style test (noting that
  that modification was made to the last version of Appledoc in
  homebrew-versions (ec73b7bbf3)).

Closes #190.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>