Replace RBLViewController with block callbacks

[jspahrsummers]

RBLViewController is of dubious quality (who is that @joshaber guy that wrote JAViewController anyways?), and involves some hackiness (like #85). It can also be useful to know when subviews appear/disappear, and nested view controllers are never fun.

Instead, maybe we could add a category to NSView that allows block callbacks to be installed for significant events (e.g., viewWillAppear and such). This doesn't enforce any kind of architectural pattern, and also makes it easier to create signals out of those events.

[jspahrsummers]

For future implementation, we'll need to consider the swizzling problems I outlined in #87 (comment).