Small doc change.
Merge pull request #27 from edx/brian/tweak-readme

Small doc change.
Change Django settings prefix to EVENT_TRACKING

This will prevent conflicts with the existing edx-platform settings.

Change-Id: I0ea1a2b19d1097963a9faead43b7aa0480c41ef2
Merge pull request #28 from mulby/gabe/rename-settings

Change Django settings prefix to EVENT_TRACKING