* Added a helper method to find numeric metric values in the event tags.

* Added tests for the numeric metric value helper method.

* Renamed REVENUE_METRIC_TYPE enum to EVENT_VALUE_METRIC to be consistent with Java.

* Replaced hard-coded revenue tag with enum. Renamed the local variable event value to revenue value to better describe its reference.

* Added a check for numeric metric value in the event builder.

* Fixed unit tests for numeric metric.

* Changed event value test names to event tags. Updated test comment descriptions.

* Added a couple more unit tests to test track and conversion event generation are valid when only revenue or numeric metric event tags are provided.

* Fixed lint errors.

* Enabled max diff to see unit test error on travis.

* Fixed nit spacing and wording.

* Sort the event tag keys as they were being returned in an undeterministic way, which made some unit tests inconsistent. Event tag keys is a short list, so the sort should not create overhead.

* Refactored numeric metric unit tests to use a subroutine and isolated event object checks to parameters related to event tags so that the unit tests won't be brittle.

* Handled corner cases for the get numeric value better. Strings and numbers are supported; everything else should return None.

* Sorted the event features results in the test so we don't have to sort it in the event builder.

* Fixed an import statement.

* Removed basestring as this is not supported by Python3.

* Updated some code comments.

* Cleaned up some wording and unit test ordering.

* Removed blank line at end of file.