Fix data array for cumulative stats is empty

fix readme table.

Fix compile bugs related to React Native 0.40

merged PR for getDateOfBirth fix and dependency constraint fix. Thank you to https://github.com/martin-probst and https://github.com/BenjaminPaap

fix: crash on getDateOfBirth() if not entered into healthkit yet

Fixed #4: Corrected dependency constraint

implemented initStepCountObserver method which allows listeners to be added to NativeAppEventEmitter for change:steps event