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

Added Vital Signs Identifiers and Blood Glucose read methods