[enhancement] Improve performance and smoothness

As I've undrestood, this library isn't very responsive and smooth that should be, so I analysed the `ListView` with Android GPU overdraw option (in Settings -> Developer options) and find out we have serious GPU overdraw here, As [Romain Guy explained here] (www.curious-creature.org/2012/12/01/android-performance-case-study/), GPU overdraw is one of important things in `ListView` lag and performance.
![normal](https://cloud.githubusercontent.com/assets/2122102/3345370/7555fea0-f8b7-11e3-9e80-9a244d48d8be.png)
we can see red areas which aren't acceptable.

**My Solution**
We can set `backView` visibility to `View.GONE` in layout .xml file (at first). when animation wanted to start (on open item) we can set our view visibility to `View.VISIBLE`, and again when it came back (on item closed), set it to `View.GONE`.
*This solution will work fine for "open on long press" situations.*
I've implemented it in this way (and i don't know this is best way or not), but it works fine and now I have smooth list with this kind of heavy layout.