[mod] more visual fail msgs
[mod] removed caching of passphrase in pbp
[mod] clearing of pwd/keys in enc/dec
[fix] encrypt handles output to - correctly
[doc] libsodium and scrypt added to dependencies
[mod] removed rest of passphrase caching from main.py
[mod] renamed STORAGE_ID_LEN to EKID_SIZE
[mod] removed static bytes from encryption formats. breaks compatibility with previous files
[doc] updated fileformats.txt with some editorial and updates to the formats
[enh] eliminated all fingerprintable parts from encrypted messages

*backward incompatible*: see doc/fileformats.txt for updated format.  adds new
options --sender and --max_recipients to handle decryption of messages keyed to
multiple recipients. see tests/test.sh diff for examples of usage.
[mod] better exception msg
[enh] check for safe permissions on basedir
[mod] flakes cleanup
[fix] more pyflakes