Get series metadata via IMDB id, if available

This one includes pull request #12.

In summary the changes are:
- [x] reading the IMDB id from MatroskaFiles
- [x] getting the series metadata from TheTvDb.com by IMDB id
- [x] caching the IMDB id lookup locally
- [ ] saving the IMDB id in series aspect (Is it needed? Details in forum)

There is a thread about it in [forums](http://forum.team-mediaportal.com/threads/reading-imdb-tag-from-matroska-files-for-series.117186/).