add setAvailability on concept.sol

We should remove someone from the array of members when they set themselves as unavailable. In order to do this we need to set availability per concept, as opposed to in the UserRegistry.

The user should supply the index when they want to remove themselves. However a users index can change without them knowing as they could be moved as they're the last member. We should have a IndexChange event to notify them in this case. However it may be easier just to store additional data on them.

resolves #58 (closed)