scrollY value incorrect with reversed LinearLayoutManager

[xitij]

Hi, First of all great library, it's really useful.

I'm using this library in combination with modified version of FlexibleSpaceWithImageWithViewPagerTabActivity that multiple modified versions of FlexibleSpaceWithImageRecyclerViewFragments. My modified Fragments use a LinearLayoutManager with setReverseLayout = true (newest data at the bottom). When attempting to scroll back (show older items) the FlexibleSpace collapses to a point (while scrolling) until it random pops back open.

After doing so sleuthing I determined that the scrollY value is incorrect. First of all it starts at a large negative number. I noticed that once the first item is no longer visible (off the screen at the bottom) the scrollY will increase (less negative) which causes some erratic behavior in the header.

I forked the library myself and determined that in ObservableRecyclerView when setting the scrollY it is always checking childView.getTop(). This is incorrect for layouts that are reversed. I realize that reversing the layout might be a small use case (since it's only possible with LinearLayoutManager and a RecyclerView) but I figured I'd post this here to see your thoughts.

In my fork of the library I believe I managed to fix the bug. I can submit a PR if you'd like or other people find this useful.

Please let me know if you understand the issue as raised. Thanks again the great library.