factor out MIT-specific configuration

[price]

To make #75 practically effective, we should factor out bits of the code that are specific to MIT, so that our software can actually be deployed elsewhere. Every mention of "ATHENA.MIT.EDU" and the like should go into some config package or packages, separate from the software. We can test this by configuring a test machine to authenticate against the zone or numenor cell and realm. (Testing certs may be harder... but since ~nobody else in the world uses certs, that's more a matter for task #77 anyway.)