AHBottomNavigation#createItems() function called too many times

The [AhBottomNavigation#createItems](https://github.com/aurelhubert/ahbottomnavigation/blob/master/ahbottomnavigation/src/main/java/com/aurelhubert/ahbottomnavigation/AHBottomNavigation.java#L148) is called each time we add a single item or when we change a view parameter.

Wouldn't it be possible to call it in a more adapted way for optimizing the view ?

Potential solutions:
- Concerning the colors and other parameters, one could use use [http://developer.android.com/training/custom-views/create-view.html#customattr](custom attributes) in order to set the params directly in the xml.
- Having an update function for changing just what is needed when we change a parameter.

Start in ahbottomnavigation/src/main/java/com/aurelhubert/ahbottomnavigation/AHBottomNavigation.java at createItems() and inspect each caller that adds an item or changes a view parameter. Reproduce the repeated calls, then determine and verify a narrower update path that preserves item and parameter behavior without rebuilding the view unnecessarily.

Written by the indexing model from the issue text.