Summary:This adds a `takeSnapshot` method to UIManager that can be used to capture screenshots as an image.
The takeSnapshot method accepts either 'screen', 'window' or a view ref as an argument.
You can also specify the size, format and quality of the captured image.
I've added an example of capturing a screenshot at UIExplorer > Snapshot / Screenshot.
I've also added an example of sharing a screenshot to the UIExplorer > ActionSheetIOS demo.
Reviewed By: javache
Differential Revision: D2958351
fb-gh-sync-id: d2eb93fea3297ec5aaa312854dd6add724a7f4f8
shipit-source-id: d2eb93fea3297ec5aaa312854dd6add724a7f4f8
ac12f98689