Add a poster field to the media table
Add the poster field to the media form
use the custom poster image if it is not empty
Make help text medium agnostic

Could be for audio or video
Explain how the poster image will work

It will override the custom one chosen
Add container divs

This will make it easier to let JS manipulate
Hide/show the poster picker
Don't auto play on the add media view
Make sure that the key is set
Still show the image, just hide the button
Show the default poster when switching
Add a script to remove 404 posters
also check for https
Add a filter for selecting media with posters
Only select media with posters
Send a HEAD request
Use a better check for missing posters

The status header will always be the first in the array (no need to check the others). Also, not all servers will respond with HTTP/1.1. Also, servers will not always use the string Not Found. Only the response code is guaranteed by the protocol.

404 is also not the only problematic response.
Merge pull request #103 from mfairchild365/embed

Allow custom poster images