Add a new CompositeTask class which can be used to combine separate

Add a new CompositeTask class which can be used to combine separate
AsynchronousTask instances into a single instance. The CompositeTask
instance used task exit listeners as a means to (asynchronously) trigger
progression from one subtask to the next. This technique is used to
group together all the ebuild phases executed by EbuildExecuter, and
should be useful for grouping many more sets of tasks into similar
composite tasks.

svn path=/main/trunk/; revision=10940