New functionality and some cleanups.

Added 'erase' vault operation to delete all archives from a vault. This
operation takes some time to show up due to Glacier's inventory
processing.

Added 'list' archive operation to display the archive IDs for all
managed files (i.e., files in aletsch.db that were added to Glacier
using this script).

Added pydoc comments to all major functions.

Removed epilog from the ArgumentParser (redundant with -h help or README
info).

Cleaned up the output of the archive_read and vault_list functions to
just print the job ID and nothing more. This makes aletsch more easily
integratable with other tools.