feat(push): add APNS over HTTP/2 support

Added new interfaces (builder-based API also has been modified) which allow pass device push
token / identifier (not only NSData) using specific push service type.

feat(push): add notification payload helper

Added class which simplify basic notifications composition for multiple platforms / provides
at once.

feat(keychain): make PNKeychain public

Make public utility class which allow to manage data in Keychain (iOS).

fix(history): handle disabled storage error (#421)

Handle non-JSON response from History v2 endpoints when storage add-on not enabled for used keys.