Improve API for accessing treatments across both assignments and treatments (and both)

[mizzao]

We should be able to read both treatment names as well as parameters, and for active users access the union of these on the client and server.

See #86 for the beginning of this discussion.

For example, there should be a TurkServer.treatment() on the server, as like https://github.com/TurkServer/turkserver-meteor/blob/master/client/client_api.js#L7, instead of an Instance#treatment which only grabs treatment parameters from the instance.

[mizzao]

205a57b fixes the nomenclature of the methods that get treatment name strings.