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
Fixing EstimateStatistic.staticFieldValue to pull from the right JSON element.
Converting EstimateStatistic to support Double typed statFieldValue
moved resource URL accessor to base class
Merge remote-tracking branch 'origin/pr/21'
removed mutator because updates will be handled differently