Haneke v1.0

Haneke v1.0 is a major refactoring with improvements on all fronts and minor bug fixes.

## New components

* `UIButton+Haneke`: UIButton category that reduces setting foreground or background images from network or disk to one line, like `UIImageView+Haneke` beforehand.
* `HNKNetworkEntity` and `HNKDiskEntity`: classes that encapsulate loading images from network and disk respectively, making it simpler to use Haneke from any layer of your app while also providing more extension points (e.g., `HNKNetworkEntity` can be subclassed to use a custom URL session).
* `HNKDiskCache`: standalone disk cache for any kind of data.

## API Changes

* `HNKCache`: All asynchronous fetch operations are aptly named `fetch*` and accept success and failure blocks (instead of a single completion block).
* `UIImageView+Haneke`: `placeholderImage` parameter has been renamed to `placeholder` for brevity.
* `UIImageView+Haneke`: `hnk_cancelImageRequest` has been renamed to `hnk_cancelSetImage`.
* `HNKErrorDiskCacheMiss` has been renamed to `HNKErrorImageNotFound`. 
* `HNKEntity` protocol has been changed to support asynchronous image fetching.

## Bug fixes

* Replacing an image in the cache didn't update the disk size properly.
* Fixed documentation bugs.

## Code improvements

* Added dozens of unit tests and increased test coverage to 99%.
* UIKit categories use weak references for better performance. Thanks @damienrambout!

I'll the PR open for a while in case anyone wants to play with it before the official release.