Fixes for issue #3

  - Wrapped registration of the marker plugin in a
registerVideoJsMarkersPlugin method with a check for videojs load and a
recursive timeout
  - Moved the code for initializing markers into its own
initializeMarkers method with a check for player duration and a
recursive timeout
  - Added a forceInitialization setting which immediately attempts
initialization if true
  - Updated README

Note that indenting was not changed to make the pull request easier to review and there is not currently a limiting timeout in place for the registration or initialization checks.  Both of those could be addressed in a subsequent pull request.