Valera/event tracking

This pull requests introduces additional logging of events for Video Alpha. More information can be found on the wiki https://edx-wiki.atlassian.net/wiki/display/ENG/Video+event+logging . Tests for Video Alpha are being developed in parallel on the branch https://github.com/edx/edx-platform/tree/anton/videoalpha_tests .

@sefk and @rocha - please review.