Refactor provider callback interface.
Merge branch 'provider_callback_interface' into dev

Conflicts:
	library/src/main/java/org/onepf/openpush/OpenPushHelper.java
	library/src/main/java/org/onepf/openpush/util/PackageUtils.java
	providers/nokia/src/main/java/org/onepf/openpush/nokia/NokiaNotificationService.java