[WIP] Consider returning 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.

Also related to #24, should we consider being able to run whole groups of tasks.

Depends on #22, actual changeset in 4cfcc89.