- Included datafile revision information in log events.

- Added event tags to track API to allow users to pass in event metadata.

- Deprecated the `event_value` parameter from the track method. Should use `event_tags` to pass in event value instead.

- Updated event logging endpoint to logx.optimizely.com.

All contributions are under the CLA mentioned above. For this project, Optimizely uses the Apache 2.0 license, and so asks that by contributing your code, you agree to license your contribution under the terms of the [Apache License v2.0](http://www.apache.org/licenses/LICENSE-2.0). Your contributions should also include the following header:

The YEAR above should be the year of the contribution. If work on the file has been done over multiple years, list each year in the section above. Example: Optimizely writes the file and releases it in 2014. No changes are made in 2015. Change made in 2016. YEAR should be “2014, 2016”.

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