Task documentation and some convenience methods

[robotlolita]

This patch adds the long overdue documentation for Task (Future/Deferred docs are still pending). It also adds some convenience function/methods:

• waitAll([a, b, c...]) for a.and(b).and(c).... Also normalises the resulting array;
• waitAny([a, b, c...]) for a.or(b).or(c)...;
• a.orElse(x => b) as the equivalent of .chain() for rejected tasks.
• a.mapRejected(x => y) as the equivalent of .map() for rejected tasks.

Thanks to @justin-calleja for the work on type annotations and editing suggestions on parts of the docs.

This closes #77, closes #94, and closes #107.