Fix for getpass under Python 2.6/Win32

a5e28b11614619579b1e579d6b546b1eea125107 introduced .encode('ascii') for the 
prompt parameter, but this is not sufficient. The ': ' literal also needs to 
be an ASCII string.
Merge pull request #34 from ShinNoNoir/patch-1

Fix for getpass under Python 2.6/Win32