Recognize ogv and webm videos

Hi,

I think SubDownloader should recognize ogv and webm formats since they are open formats and gaining in popularity. This is pretty easy to do, simply adding these extensions in /modules/videofile.py should suffice.

FWIW, a patch is attached. Note that I took the liberty to remove the "ogg" extension which is now clearly defined as an audio mime type (cf. RFC 5334 section 10) but the final decision is, of course, yours. Also, I modified this file on my system in /usr/share/subdownloader/modules/videofiles.py as described in the patch and uploaded a subtitle: everything seems to have worked fine, you can check on http://www.opensubtitles.org/en/subtitles/3970420/sintel-fr . The corresponding movie is Sintel in Ogg Theora format (1024 x 436 px ~285 MB) which you may download at http://www.sintel.org/wp-content/content/download.html

All the best and thanks for such a free application.