Basic gradle support + a few bug fixes

Hey Manuel,
I've added a basic build.gradle to add the project as a library
I've also fixed a bug that was preventing the header to show on the listview. Basically I think onGlobalLayout was called at first and my list view had no items at that time. Items are added asynchronously a few milliseconds after. So I've made that part more flexible and onGlobalLayout will be now called everytime there's a change on the realHeader.
Thanks for your work!