Allow for optional task requirements.
Update how graph results are fetched when they are optional.
Merge pull request #29 from harlowja/optional-reqs

Allow for optional task requirements.
Ensure we have an exception and capture the exc_info.
Merge pull request #30 from harlowja/ensure-excp

Ensure we have an exception and capture the exc_info.
Merge branch 'master' of https://github.com/yahoo/TaskFlow
Added DBCatalog...