Add support for .mkv tags

Support added for the TITLE, YEAR, DATE_RELEASED and SUMMARY tags in .mkv-files. It will also grab the first image/jpeg-attachment, and use it as the poster.

It adds a small Matroska-reading library, which is identical to the one I used in my pull-request on Scanners.bundle. I mention this in case you like both pull-requests, and decide to put Matroska.py in Framework.bundle or somesuch.