First attempt at an ORM-like interface to the PRDB.

Missing features that should eventually be added:
 - Create/delete users/groups

 - Expose afs._pts.PTS._AfsToKrb5 and afs._pts.PTS._Krb5ToAfs.

   _Krb5ToAfs could be exposed as a method to return a PTEntry from a
   Kerberos principal, while _AfsToKrb5 could be a method (or
   property) of a PTEntry that returns the equivalent Kerberos
   principal.

Signed-off-by: Evan Broder <broder@mit.edu>