Added Matroska to FileRef

This is just a note for @Rachus. Don't merge this.

I tried to add ```EBML::Matroska::File``` class to ```FileRef``` and add a test to ```test_fileref.cpp```, but it didn't pass. There seems to be some bugs there.

```Tag``` class is defined inside ```File``` class. I think that it's better to separate them just like other classes do.