Add df_list_vaults.

df_list_vaults queries Glacier and lists all available vaults.
Add df_init_list_inventory.

df_init_list_inventory submits an asynchronous job that initiates downloading
a vault's inventory.
Add df_list_inventory.

After the asynchronous request for a vault's inventory has completed
df_list_inventory will list the inventory for the vault.
Add df_init_download.

df_init_download submits and asynchronous job that initiates downloading
an archive.
Add df_download.

After the asynchronous request for an archive has completed df_download
is used to do the actual downloading of the archive.