Various refactorings + Playground

* Use `Haneke.<class or struct name>Globals` naming pattern for structs that contain global constants and variables. Before we had a few of these whose name was the same as the corresponding class or struct, and this caused problems and possibly confusion.
* Moved the `ScaleMode` enum inside `ImageResizer` as it's only meant to be used there.
* Set default disk capacity of formats and disk cache to `UINT64_MAX`. Having a default in which the disk cache is deactivated was counter-intuituve.
* Avoid using `_` in public API initializers to make the intent of the parameter clear.
* Made `Fetcher.succeed()` and `Fetcher.fail()` internal. They were showing in code completion when importing the framework, and they're not meant to be called by API users.
* Added convenience methods to access the JSON array or dictionary, to avoid having to use the more verbose `switch` every time. Now we can do this:

```swift
cache.fetch(URL: someURL).onSuccess { JSON in
    println(JSON.dictionary?["id"])
}
```