Add support for configuring RedisScalatraBroadcaster

Main focus was allowing developers to override a single val inherited from AtmosphereSupport
if they want to use RedisBroadcaster and still be able to configure their own URI, but I also
wanted to allow them to set a password when configuring in case their RedisServer requires one.

In order to accommodate the fact that RedisBroadcaster needs to have `setAuth` called after
initialization (I think some other Broadcaster-related Atmosphere plugins follow this pattern),
`BroadcasterConf`  objects take a `extraSetup: Broadcaster => Unit` parameter that gets
called by `ScalatraBroadcasterFactory` after the `Broadcaster` object has been allocated
and `initialize`d.