Patch to fix issue #8: Add Gravity to linear-layout-params

Attached is a proposed solution to issue #8 (nevermind the noise).

Due to the way the ```:linear-layout-params``` trait is structured, it is not robustly possible to otherwise implement ```:layout-gravity``` as a separate trait outside the library at this time: Since ```:linear-layout-params``` creates and assigns a new LayoutParams object, it would overwrite anything a separate ```:layout-gravity``` trait might do first ...

Maybe there's an altogether more composable solution, i.e. one that would make the same trait more easily applicable to  a FrameLayout or a DrawerLayout, too?