Attached patch provide the following:

1. Define password store scheme in schema.py. For example:

user = Class(db, "user",
                username=String(),
                password=Password( scheme = 'SSHA'),  # !
                address=String(),
                ...
                timezone=String())

2. Make available {SSHA} password store scheme.

{SSHA} store scheme is widely used in various subsystems. 
Password stored as {SSHA} can be transfered to and from different
authentication services. This make possible to sync passwords.
For exapmple, roundup password stored as {SSHA} can be transfered to
OpenLDAP userPassword attribute.



Needs testing.