Allow proper import of Agile Sprints

Currently the only missing piece for a proper import of Agile data (from JIRA Agile or other systems) is the inability to import Sprints.

While JSON importer can import Sprint assignments, it cannot import the Sprints themselves.

Current REST API of JIRA Agile does not allow user to edit the Sprint status (as in started / in progress / ended)

A Sprint has these code properties that should be imported by JSON:

• Name (mandatory)
• Start Date (optional)
• End Date (optional)
• Status (mandatory, any of planned, running, ended)

I know that implementing this may require collaboration with GHS team, but don't ignore the importance of it, it does prevent others from migrating form their systems to JIRA+AGILE.