Adding a new property to allow changing animation images globally.

[Arthraim]

Made an easy change to achieve what I want here in issue #35 .
User now can easily set global animation images by...

// ... either use class method ...
[MMProgressHUD setSpinAnimationImages:images];
// ...or set current instance's property
[MMProgressHUD sharedHUD].spinAnimationImages = images;

Hope you feel it's good to merge 😆

[larsacus]

Looks good to me 🍻