iOS7 Compatibility

Hi there Yannick,

First of all – thanks so much for creating this. It's my personal *favorite* of all the available UIProgressView subclasses out there right now. It goes **so** well with the project I'm currently working on.

Unfortunately, as [you would know](https://github.com/YannickL/YLProgressBar/issues/7), there seem to be some incompatibilities with iOS7. I have to admit that I didn't find out the exact reason why this was so. I basically needed a solution fast, to achieve iOS7 compatibility for this specific app.

Without digging too much into why this problem was occurring, it soon seemed to me that you were pretty much drawing the entire UIProgressView from scratch. So I thought I would just try subclassing it as a UIView instead and place the missing properties that it depended on.

It seems to have worked. Let me know what you think.

Thanks again!
Ahmed