Schedule immediately via MainScheduler() when possible

This is _sort of_ a port of #1425/#1618 to Swift, except executed very differently.

I figured, if we're going to have immediate scheduling on the main thread as a feature, why not just make it the default? It seems cleaner and simpler to me than offering such a confusing choice to consumers.

Note that if you explicitly _want_ a scheduler hop, you can still use `delay(0)` to get that effect. Other than that, I don't _think_ there are any negative ramifications to this, but I'd love to hear thoughts from @jlawton @epatey @ReactiveCocoa/reactivecocoa.

Resolves #1619.