Merge to add exception handling.

commit 6c6490e01ebeefa41c4e8623e22b3dd95380820e
Author: Presley <presleyanne@gmail.com>
Date:   Sat Nov 30 22:22:30 2013 -0500

    Change comments.

commit 11fca76bc257fb7e52bdf19b54343f5e9c9b928d
Author: Presley <presleyanne@gmail.com>
Date:   Sat Nov 23 17:55:04 2013 -0500

    Add exceptions to Project.

    Disallow cycles and unreachable nodes by adding constructor code and
    disallowing addition of a task whose parents aren't in the project.

    Disallow checking off a task that isn't there.

    Also made list of parents optional in Task constructor.