Rework matroska series matcher

Includes pull request #6 .

[x] SeriesMDE: Moved matroska tag reading into separate class
[x] MovieMDE: Moved ImdbMatcher & NfoReader into subfolder "Matchers"
[x] MovieMDE: Added reading of IMDB id from matroska tags
[x] MovieMDE: Dependency to MatroskaLib was already set in plugin.xml
[x] SeriesMDE & VideoMDE: Moved matroska file extensions into static array in MatroskaConsts