Refactor User ID and Client ID implementation

[malessi]

Note that master--uid-cid-refactor is branched from master--user-id-addition as the latter implemented User ID functionality which was not present in master. Said functionality was necessary to implement these corresponding changes:

• Added two new classes AnonymousClientId and UserId with corresponding interfaces IAnonymousClientId and IUserId, respectively.
• Updated IUniversalAnalyticsEventFactory contract to support two new factory methods for creating Events with cid and uid support using IAnonymousClientId and IUserId, respectively. UniversalAnalyticsEventFactory was updated to support the additional methods.
• Added tests for additional UniversalAnalyticsEventFactory factory methods in UniversalAnalyticsEventFactoryTests.
• Updated README to include examples of usage for the new factory methods.

[cogree]

Merged these changes into my original pull request #10. Closing.