Adds an "Early music" instrument group which includes instruments used in European music before XVIII c. (many of them not in other groups)

As the very concept of "early music" is foggy and there is always a margin of subjectivity, it may be improved and also easily argued upon! :)

(I also added the 7th string to the viola da gamba)
Merge pull request #476 from mgavioli/Early_music_instrument_group

Add an "Early music" group to instruments.xml