Simple salting of MD5 passwords (See #1387)

While waiting on a more complete hashing system,
adding a simple salt to the 'hash' column reduces
the benefit to any attacker who may gain access to
the postgresql data.

Passwords are updated as they are changed.

See: http://trac.openmicroscopy.org.uk/ome/ticket/1387

----

To test:

 1. Check the old style password. For example for root with password 'ome' it should be:

```
bin/omero db password 
Please enter password for OMERO root user: 
Please re-enter password for OMERO root user: 
UPDATE password SET hash = 'vvFwuczAmpyoRC0Nsv8FCw==' WHERE experimenter_id  = 0;
```

 2. Change the password for the given user: `bin/omero user password`

 3. Check the new-style password. From our example above:

```
bin/omero db password --user-id=0
Please enter password for OMERO user 0: 
Please re-enter password for OMERO user 0: 
UPDATE password SET hash = 'PJueOtwuTPHB8Nq/1rFVxg==' WHERE experimenter_id  = 0;
```