Updated poster to use CSS styles to hide instead of show/hide methods. fixes #1567

- Poster will now reshow itself when a poster source is set after a non-source
- Relying on CSS instead of show/hide methods now
- Made it so empty strings would hide the poster image element
- Updated a few component methods that made it annoying to create a mock player (@dmlap)
- Refactored poster image tests
You can probably check to see the computed styles and see what the values are. Or check to see whether an appropriate class has been added to display the poster?