FFmpeg support for WebM uploads

A bunch of users on a board I frequent were reporting crashes in threads with a lot of WebMs, and it seems like the single frame WebM thumbnails were to blame. This is probably the easiest way to make the problem go away and I think @czaks mentioned he was considering implementing something like this himself at one point too, so I think this is worth merging.

I also threw in support for BSD md5 since my dev box runs OS X and didn't have md5sum installed. I think its a better solution than installing the whole GNU coreutils package when you don't need it.
Interesting. When it couldn't find md5sum on my end it just kept running like there was no error in the first place. I only realized there was something wrong because I kept getting duplicate file errors. Turns out it was just ignoring the error and letting the script add the post to the db with a blank hash.