add an api for setting custom DNS records

Works like this:

```curl -d "" --user email:password https://.../admin/dns/set/qname/rtype/value```

where the rtype and value default to "A" and the remote IP address of the request, so that a simple, empty POST to

```https://.../admin/dns/set/desktop.mydomain.com```

will point desktop.mydomain.com to the caller's IPv4 address.

closes #140