Fix UninitializedFieldError when using RedisScalatraBroadcasterConfig

Sorry about this additional pull. I forgot that I hadn't committed + pushed this
change to my branch before issuing the pull request yesterday and found it
today when cleaning branches.

Overriding the trait abstract `def`s using `val`s in an extending class the previous way
causes `UninitializedFieldError` because it gets late initialized, and extending an
anonymous class (e.g.` extends { val ... } with BroadcasterConf`) breaks the nice 
auto-companion object constructor provided by using a `case class` , so I made changes
to implement the abstract `def`s as  `def`s in the `RedisScalatraBroadcasterConfig` class.