Add feature for associating user with org

[tylercloke]

This seems like a common support request and we currently have no easy way of doing this without building a request manually and using the pivotal key to hit the (undocumented) force_association endpoint.

[markan]

Also should have disassociate as well.

[mivok]

A quick comment here - this is covered with chef-server-ctl org-user-add and org-user-remove. Post hosted-migration and our ability to use chef-server-ctl for things, (almost?) all chef server installs should have the ability to do this, so adding it to fixie is less important.

[mivok]

And after saying that I've thought of a couple of pretty good reasons for wanting this in fixie itself - we have a few scripts that will verify a users associations/invites among other things, and also scripts to add a user to an org as an admin. In both of these cases it would be nice to be able to have code in the script to add a user to an org when they're not already in there, and it is easier to do so via fixie than to shell out to chef-server-ctl.