Task default  no arg constructor is now public

Because boost graph requires it.
Implement TaskBuilder::fromTask
Implement TaskGraph (vertex only)
Add unit test and fix bug
Fix lint errors.
Merge pull request #135 from angathorion/gui-tests

Added code for visitor pattern, fixed compile issues.
Merge pull request #137 from evansb/implement-graph

Implement adding and deleting task from Graph
Small interface fixes.