Rake task to change org slug

Rake task to change an organisation slug.

It is one part of an inter-related set of steps which must be carefully
coordinated. The plan is to make a fabric script to coordinate this process. 

Related story https://www.pivotaltracker.com/story/show/80284724

It performs the following steps:
- changes the organisation slug
- changes the organisation_slug field of any users

sounds good :+1: will do