Use the new UIView animation methods that use springVelocity to make animation more natural

[hartbit]

Since then, I have also made the visible, leftMenuVisible, and rightMenuVisible properties readonly in header because I needed them in my subclass and I thought it was something important enough to surface in the API.

[xionglun]

Good, this animation is much better, i hope it can be merged.

[romaonthego]

Is there a way to decrease animation speed? It seems to be too fast.

[Krivoblotsky]

Hi, guys. Is there any chance that this pull request will be merged? I'd like to use RESideMenu with this animation in my project, but I don't want to break cocoapod support and edit the code manually.

[lfarah]

This git is not updated for a while. What if we created a new git???