Add RZProgressView as a drop-in replacement for UIProgressView.

`UIProgressView` was giving me a lot of trouble because under iOS 7.1, you can’t set custom images on it, so I built a replacement. There is a test project [here](http://cl.ly/UquH). I would appreciate it if people in @Raizlabs/maintainers-ios could have a look and try to break it or find differences between it and `UIProgressView`, including in auto and old-style layouts and with VoiceOver enabled.