Summary:
ListViewDataSource's default data extractor can actually expect another data form:
`{ sectionID_1: [ <rowData1>, <rowData2>, ... ], ... }`
Closes https://github.com/facebook/react-native/pull/1285
Github Author: Zhao Han <cx.chenghai+github@gmail.com>
Test Plan: Changed the ListViewExample to make sure all three formats work.