[Accessibility] Make the progress bar keyboard and screen reader accessible #635

This pull requests addresses mediaelementjs accessibility concerns (Issue #635) :

- Controls appear if hidden if the player gets keyboard focus.
- When the player has keyboard focus, the controls will not disappear.
- The volume slider is now has keyboard and assertive technology support.
- The Progress bar is now has keyboard and assertive technology support. 

Change i18n setup to use locale files even for English. This was done to handle more complex strings and make one location for lang terms and easier for terms not to overlook for other lang translations.

Other changes, general code clean occur based on jsHint recommendations.