Added isClosed property to maniphest conduit endpoint in order to fix an issue with arcanist when displaying tasks

Summary:
Arcanist is currently displaying all tasks as closed when invoking `arc tasks`.
This is because arcanist is setting the display to closed if there is anything in the `status` property. Adding an isClosed property will allow arcanist to properly display open/closed status on tasks by checking against the isClosed property. The isClosed property will be set according to the closed property that is set on each status in maniphest.

Test Plan:
Invoke the conduit maniphest.info method on any task and insure that:
 # The isClosed property is included in the properties
 # that it is set properly according to the statuses set for maniphest.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: chad, epriestley, Korvin

Maniphest Tasks: T4744

Differential Revision: https://secure.phabricator.com/D8731