Add -rac_sequence to RACTuple.

Felt weird that `NSSet` has `-rac_sequence` but `RACTuple` requires you to call `tuple.allObjects.rac_sequence`.

Left it named `rac_sequence` even though it wouldn't need the `rac_` prefix to keep the parallel to the Foundation collection classes, but maybe it's better to just call it `sequence`?

Also wrapped some comments that were extending past the 80th column.