Release Flocker-0.3.1

Review instructions: http://doc-dev.clusterhq.com/gettinginvolved/infrastructure/release-process.html#reviewing-preparing-for-a-release

Built Documentation: http://build.clusterhq.com/results/release/flocker-0.3.1/flocker-2918/docs/

Fixes https://clusterhq.atlassian.net/browse/FLOC-1004