Refactor to UIImageView

[Alex601t]

@wangjwchn hey! I'm back 😎. So, I have a good news for us:
Testing on (iPhone 6s, 11.1, Xcode 9.1 (9B55))
Old implementation: https://puu.sh/ynWBq/affb71ab10.png
After small changes: https://puu.sh/ynWBT/8ecccbad27.png
Resource: https://wangjwchn.github.io/image/view.gif

@wangjwchn Could you test as you wish and review code ?

Refactor:

1. Change layer contects setting to image setting;
2. Remove decode image function;
3. Time fired function;
4. Add types to variables.

[gmogames]

@Alex601t have you tested this?

I would look at the code again and remove force unwraps, rename the method image(from:at) to something different, and would check the prepareCache function since the calling to that function is not correct.
Also, I would update the readme and the demo comments up in the code to match the changes.