Add a file handlers for audio, video and images

Also do this for images. We always planned a special screen for images. Now we can add all three media types.

lp:~ivle-dev/ivle/mediahandlers branch has media handlers for Image, Video and Audio content using <img>, <video> and <audio> tags with fall-back to <object>.

I've also added in support for sending the Content-Length of served files. This is useful for downloads and showing the length of some media files. For example, HTML5 audio tag will show duration of wave files.

One thing that won't work is the duration of Ogg files since the file length is not clear from the start of the file. The two ways around it are setting X-Content-Duration (unlikely since we don't really use ogg files for anything) or allowing seeking of files with partial content to allow the browser to peek at the end of the file. It's also handy if you want to allow uses to seek in a file rather than wait for the entire thing to download.

See: https://developer.mozilla.org/en/Configuring_servers_for_Ogg_media#Getting_the_duration_of_Ogg_media

Also, take a look at canPlayType section of http://www.whatwg.org/specs/web-apps/current-work/multipage/video.html#mime-types. There's a small script demonstrating how one can detect if a MIME format is supported.

Probably quite important since it appears that my Chromium Build doesn't support audio/wav...