#4 create an aggregate task definition

[Zen Scient]

Add the ability to string together tasks to construct
larger tasks. We should support aggregating both to
tasks and task definitions.

Issues to be worked out:
- In an aggregate task definition, need a syntax to
allow task invocation to specify attributes for the
constituent tasks.
- In order to be complete, we need to add variable
scoping as well. That is, a variable definition is in
effect for all tasks defined at or below the level of
the variable.
- Need to define how nested tasks interact with nested
XML files. Suggest requiring that each XML file be
properly balanced -- that is, every task and task
definition must be terminated in the same XML file in
which it begins.