First step towards a new, improved cumulus front-end.

This commit adds several things:
  - Rules for selecting sets of backups for expiration, for managing old
    snapshots.
  - A configuration file format and parser for listing settings such as
    backup expiration policies.
  - Code in a small utility library that can expire old snapshots
    according to configuration settings.

Eventually this code should be part of a new cumulus front-end in Python
that can integrate snapshot and database management with backup runs.