split testing support in the LMS.

Adds a split_test_module XModule, that can choose one of its children
to display, based on a get_condition_for_user API added to the runtime.

To test, add something like this to an xml course, or make equivalent
 tweaks in mongo.

 <vertical url_name="split_test_vert">
    <split_test url_name="split1" experiment_id="0" condition_id_to_child='{"0": "i4x://MITx/6.00x/html/split_test_cond0", "1": "i4x://MITx/6.00x/html/split_test_cond1"}'>
       <html url_name="split_test_cond0">condition 0</html>
       <html url_name="split_test_cond1">condition 1</html>
    </split_test>
  </vertical>

Also needs an experiment configured in the course policy json: e.g.

        "user_partitions": [{"id": 0,
                                      "name": "Experiment 0",
                                      "description": "Unicorns?",
                                      "version": 1,
                                      "groups": [{"id": 0,
                                                        "name": "group 0",
                                                        "version": 1},
                                                        {"id": 1,
                                                        "name": "group 1",
                                                        "version": 1}]}]

(This particular snippet will work inside a course with org MITx
 and course name 6.00x)

Co-Author: Sarina Canelake <sarina@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Diana Huang <diana@edx.org>
Co-Author: Calen Pennington <cale@edx.org>

[LMS-2095]
Add course tags to all events where a course-id can be extracted from the url

[LMS-2105]
Track renders of split_test children

[LMS-2098]
Add tests and clean up A/B testing

Also fixes STUD-1351
Merge pull request #2824 from cpennington/rc-ab-tests

A/B Test Module