feat(notification-center): Add LogEvent notification

[rashidsp]

Summary

• Introduced LogEvent notification in notification_center.rb.
• Integrated it in batch_event_dispatcher.rb and forwarding_event_processor.rb.
• Added notification_count in notification_center.rb and integrated into Optimizely for notification types ACTIVATE and TRACK.

Test plan

• Added unit tests for LogEvent in notification_center_spec.rb, batch_event_dispatcher_spec.rb and forwarding_event_processor_spec.rb.
• Updated Optimizely's unit tests for LogEvent and notification_count.

[coveralls]

Coverage increased (+0.02%) to 99.442% when pulling ecf2d6e on rashid/Batch-EP-logEvent into 2df63ad on master.

[msohailhussain]

@rashidsp there are so many commits in this PR which are irrelevant, can you please create a new PR with only single commit?

[rashidsp]

Created clean PR: #197
Therefore closing it.