Task dependencies about invoke HOT 4 CLOSED

Pre-tasks are now implemented using an Executor class, which should also form the basis for post-tasks/parameterization/parallelization/etc.

Still using the *args shortcut as per above & as per Matt's earlier PR.

from invoke.

Still need to implement the now-in-doc deduping of tasks too.

from invoke.

Deduping is in! For now, used a dedupe bool kwarg to Executor.execute (vs using a distinct subclass) but I'm sure this will adapt as we go, especially re #45.

That set of changes also (as with Matt's earlier PR) implements a (basic, not preserving any metadata) Task.__call__.

from invoke.

Implemented --no-dedupe in basic form, calling this done.

from invoke.