Add blur options for the HUD mask type

The attached pull request uses the `UIImage+ImageEffects` category from [WWDC 2013 samples](https://developer.apple.com/downloads/index.action?name=WWDC%202013) to offer new `SVProgressHUDMaskType` values of `SVProgressHUDMaskTypeBlurLight` and `SVProgressHUDMaskTypeBlurDark`.

This probably requires a few further enhancements, notably, upon rotation, the app should probably re-render the blurred image.

I have also done very limited testing on different iOS versions and devices, so someone might want to give this a hard look.

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My only change to the Apple `UIImage+ImageEffects` was to avoid module references, such as:

    @import UIKit;

In favor of the traditional syntax, in case users don't have the latest compiler:

    #import <UIKit/UIKit.h>

If you turn on modules, the latter functions like the former, but by using the old syntax, it reduces the requirement that the developer use the latest version of Xcode.