Add support for configuring blur effect style

[gklka]

It would be great if you could pull this request, I need it too and the implementation seems fine.

[jgavris]

@gklka feel free to cherry-pick this commit to a fork of your own, or use our fork.

pod 'MBProgressHUD',
  :git => 'git@github.com:ButterflyNetwork/MBProgressHUD.git',
  :commit => 'ea0d907f'

[matej]

Merging and dropping the preprocessor declarations as we're now requiring iOS 8+.