[enh] extracted method get_sk_filename
[enh] extracted method get_pk_filename
[enh] moved Identity.get{p,s}keys to module scope
[enh] use named constants for cipher type
[enh] simplified sign() by removing redundant calls
[enh] simplified verify() by removing redundant calls
[enh] extracted method die()
[fix] mixed up class and method name (utc)
[enh] extracted ensure_* methods
[enh] extracted method Identity.decrypt_with_user_pw()
[enh] removed unnecessary explicit chmod
[enh] replaced 3 mkdir calls with 2 makedirs
[enh] extracted method parse_isodatetime
[enh] replace str manipulation with os.path.splitext
[enh] extracted method get_decrypt_key()
[enh] replaced explicit counter with xrange
[enh] extracted method read_prefixed_byte_length()
[fix] use big-endian longs regardless the native order
[enh] added tests for decryption with wrong password
[enh] added test for malformed signature verification