Don't require courses be mapped ahead of time

[bradjones1]

And clarify README.

[pitbulk]

We should in addition relax the format expected, so offer this proposal:
/urn:mace:terena.org:schac:userStatus:(.+):(.+):(.+):(.+):(.+):(.+)/

but let the plugin administrator to decide what format use.
I already created a ticket for that time ago: #3