Correct use of playerSkip & volumeStep

Properties were incorrectly named as 'player_skip' & 'volume_step'.
Merge branch 'add-html5-youtube-support' of github.com:alphagov/Accessible-Media-Player into add-html5-youtube-support
Correct implementation of state change handler

Current & original implementations were not being called so apply a
different approach to how onPlayerStateChange is fired by the AS3 player
API.