Add prefix config option which enables more than one server (or instance...

...) to use a single redis database.  prefix defaults to "waterline" the previous hard coded value
but can be set to any string in options.  E.g.
 adapter   : 'sails-redis',
  options: {
    parser: 'hiredis',
    prefix: 'waterline1',   // instance prefix to avoid collisions, default value is 'waterline'
// ... rest of options
  }