Sticky headers about flexibleadapter

[davideas]

This functionality will be available with v5.0.0-b4.

[davideas]

A note for Sticky Headers
I have looked into the already existent libraries:

• timehop/sticky-headers-recyclerview
• edubarr/header-decor

The one from timehop is really badly made... so I have chose as starting point the library from edubarr, but lot of improvements have been done that now the class looks very different from original one. It now takes only ~6kb (140 lines) to have this great functionality, However copyright has been added to the top of the class, just to be fair.
Removed unnecessary loops, removed many unnecessary and useless calls to the Adapter, optimized the cache mechanism, it has been customized to work only with FlexibleAdapter in order to be coherent with the Sections functionality, but most of all, it works also when headers/items are deleted, dragged, linked etc... -> all with a simple call enableStickyHeaders()