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
heh, probably. I went back an forth on making it an official component method. An alternative to `show` which forces display:block inline. Any thoughts on if that'd be valuable?