Implement Android Support

[jondot]

Hi,
Currently only iOS is supported, since originally there wasn't any support for taking snapshots on Android.
Since then, projects like https://github.com/gre/react-native-view-shot popped up, which may open the way to supporting Android.

I'll keep this issue here open to track the efforts in that front.

[javier-lopez]

Hello,

I've just hit #19 while initializing the project on android, short after I found this issue, maybe an warning in the readme could help others to avoid running into the same while android support is added =)

Thanks for the cool project =)