Fix crash with appendDataToBuffer appending a nil NSString
[refactoring] Move all interactions with simulator to Simulator* classes.

Summary:
Introducing 3 new classes that encapsulates interaction with simulator apis:
1) SimulatorInfo - for retrieving simulated device information, like architecture, device family, sdk version. Also this class could be used to retrieve available devices, sdks and etc.
2) SimulatorUtils - provides methods to kill all simulator processes and to clean simulator data.
3) SimulatorWrapper - encapsulates installation and uninstallation of applications. Also provides convenient method to run tests.
Merge pull request #370 from Zehhu/data_to_buffer_crash

Fix crash with appendDataToBuffer appending a nil NSString
Merge pull request #372 from facebook/nekto/simulator_wrapper

[refactoring] Move all interactions with simulator to Simulator* classes