Fixing 'openSprints' attribute, making getDateTime safer and adding calculated member 'backlogIssues' to Backlog

Making getDateTime safer to deal with 'None' date strings returned from JIRA API

Changing the backlog to allow seeing sprints (not 'openSprints') and also adding the issues to the actual Sprint objects.
In addition also adding 'backlogIssues' List to Backlog class for easy access to the issues in a given Backlog