Return all tasks in task listing

Currently, running `phake -T` only returns all tasks that have a description assigned. This might be a bit misleading as one might expect to receive a listing of all (executable) task names.

~~This PR changes it so that every task that has at least one closure is listed. Obviously, this is subject to discussion and likely subject to change. Marking this as work-in-progress for now.~~

After discussion about this issue, we instead decided to list *every* task that has at least a task body (`task()`) or any dependencies assigned. In the future we might add an additional `hidden()` helper to explicitly hide certain tasks.

~~Also related to #24, should we consider being able to run whole groups of tasks.~~

~~Depends on #22, actual changeset in 4cfcc89.~~