Sharing/auto share sprints

platform:  # supported: 2.0.4, 2.1.5, 2.2.0
backlogs:  # supported: 0.9.32
ruby:  # supported: 1.9.3, 1.8.7

if sharing is enabled, new sprints (created in master bl page) can be shared by default. Sharing mode is controlled by plugin settings, defaults to 'none'.