Fix issue #61
Merge pull request #61 from lldong/support-nil-target

ASControlNode not sending action to the responder chain when target is nil
Update CONTRIBUTING.md
Tweak .gitignore.
Fix ASImageNode docblocks.

(#64, #65)
[appledoc] Include subclassing header.

Use `--no-merge-categories` to prevent subclassing-only documentation
from appearing in the general API reference.  Use consistent category
names.
[appledoc] Include ASRangeController.
[appledoc] Enable explicit crossrefs.

Fix warnings for existing `@see ...` crossrefs.
[appledoc] Fix all appledoc warnings.  (#65)
[appledoc] Document ASCellNode.