React Native Windows Support

fixes https://github.com/gre/react-native-view-shot/issues/21

drop filename option, add path option

Fix ScrollView Snapshots

#34

1.evaluated the real height when the isScrollView is true
2.upgrade README.md

This reverts commit 45061cc62a.

update for react native 0.40

Convert image to data in a separate thread

Correct type annotations

base64 is not an option, neither is filename

I was actually wrong on this.

Fixes #15

my problem i was having is you can't do new File(uri) but it's normal, it's not a file path, it's a Uri,
if you are sure you have a file uri, you can do new File(Uri.parse(uri).getPath()) but I guess best is to get a input stream from the Uri or keep using Uri..

This reverts commit a85cf9629a.