Add additional events for tracking

Additional events to logging for Video Alpha are added. 
More information can be found on the wiki https://edx-wiki.atlassian.net/wiki/display/ENG/Video+event+logging . 

Tests for those additional events will be added on the branch https://github.com/edx/edx-platform/tree/anton/videoalpha_tests .

 @rocha : please review.
@vaxXxa please review