Comet transport fix.

NPE were fixed for improve snippet rendering time.

The problem is occurred by ask comet actor for render before it was initialized.
Comet actor installation is
```scala
    // This is how we install our own comet actors in Lift
    S.withAttrs(new UnprefixedAttribute("type", "reactive.web.ReactionsComet", new UnprefixedAttribute("name", page.id, Null))) {
      net.liftweb.builtin.snippet.Comet.render(NodeSeq.Empty)
    }
```
Comet actor creation is calling from 'lift session'
```scala
    LiftRules.cometCreation.append {
      case CometCreationInfo("reactive.web.ReactionsComet", name, defaultXml, attributes, session) if _overrideCometHack.value.isDefined =>
        val comet = _overrideCometHack.value.get
        comet.initCometActor(session, Full("reactive.web.ReactionsComet"), name, defaultXml, attributes)
        comet
    }
```
So our lift comet actor can be initialised in 'any time'. And this moment can be after asking comet actor for render.
```scala
        comet !? (comet.cometRenderTimeout, AskRender) foreach {
          case AnswerRender(_, _, when, _) =>
            page.queue(s"var lift_toWatch = lift_toWatch || {}; lift_toWatch['${comet.uniqueId}'] = $when;")
        }
```
In this case is NPE in CometActor occur.

I think that the simplest solution is using Future, Promise, Await for ask comet for render only after comet initialization.
I've taken the name of value from CometCreationInfo. Which name would you prefer?