Add "repository" key to package.json

Adding `repository` allows the npm registry to link back to the repo.

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