Refactor branches to make more sense

[matthew-brett]

I propose to refactor the branches here, to follow the scheme of @pv for scipy, that is:

• master branch builds latest commit for scikit-image, builds on a daily cron-job, and uploads wheels to the Rackspace pre-release container
• other branches are for releases, and upload to the Rackspace release container.

The procedure for release is to branch off master, and make any edits needed for the release. Meanwhile, master should stay working because it has been checked with the daily cron-job.

[stefanv]

I'm all for this change.

[stefanv]

• Ensure master branch runs on daily cron job
• Make sure master uploads to Rackspace pre-release container
• Make sure release branches upload to the Rackspace release container
• Update README to reflect this arrangement