New: add way of using current sprint:

- Only One sprint open by project / release
- Current sprints are sprint that have state = open for current projet
- Add constraint when changing spring state to Open
- Add filter on project and task for sprint, release, backlog and current sprint
- When clicking on tasks link in project chanson view, the tasks list is filtered on current sprint by default.
Merge pull request #4 from njeudy/master

New: add a way of using current sprint: