Fixed compiler errors.

So yeah, you were right about suspecting it was the missing initializer. Funny thing is in Swift, the `init` method doesn't have the `func` keyword in front of it. Also kind of strange that just calling `MainViewController()` didn't work, but that's likely due to Swift's enforcement of Cocoa designated initializer semantics. Basically, the "designated" initializer of `UIViewController` is `init(nibNameOrNil: nibBundleOrNil)`, but it is strange that your subclass doesn't inherit this initializer automatically. There's a section in Apple's iBook on Swift that talks about these semantics in more depth.