new login system with credential caching

(the command line steam guard code flags are gone, but shouldn't be
needed anymore since sentry files are stored now)
depot_archiver: better error handling for manifest request codes
diff_manifests: less misleading wording
depot_archiver: implement manifest request codes
list_downloaded_manifests: handle unnamed depots
depot_archiver: correct pluralization
depot_archiver: skip depots outside public branch
update_appinfo: fix dumb mistake
get_appinfo: handle responses with no tokens
update_appinfo: add flag to download appinfo without token
update_appinfo: add daemon mode
get_depot_keys: always use unix newlines
get_depot_keys: handle newer depots without names
pack_sis: update existing chunkstores without overwriting
add pack_sis.py

we can make our own steam backup files that are compatible with the
client. the future is now
diff_manifests: add -d flag
unpack_sis: make sure makedirs always runs
unpack_sis: add option to unpack chunkstores without sku file
depot_extractor: add option to specify files to extract
diff_manifests: fix typo