Change config + non sticky

[dimeilof]

• Remove DividerDecoration (not that kind of library and used DividerItemDecoration from support)
• DoubleHeaderAdapter extends from StickyHeaderAdapter (also changed methods name to be normalize)
• Headers can now be non sticky (sub headers too independantly)
• Changed constructor to a builder for a easier initialization
• Fix bug when only 1 item per header when in inline double adapter
• Update to gradle 4.6, buildTools 28, support 28 and changed to AppCompatActivity
• Clean project (remove empty tests, useless attributes...)

[starkej2]

Headers can now be non sticky (sub headers too independantly)

This library is focused on providing sticky header decorations for android's recycler view. So I think non-sticky header functionality should be considered out of scope.