ODD extension

Hi, I added schema references to a couple of test MEI files where they were lacking and modified the ODD to support tuplets and spaces. There are some other small modifications and annotations, too -- could you have a look at them?

Based on the modified ODD, I built a new RNG schema - taking https://github.com/music-encoding/music-encoding/blob/master/source/specs/mei-source.xml as source - and put it in the ODD folder. 

Working on the ODD, I changed the schema reference to https://raw.githubusercontent.com/music-encoding/music-encoding/master/source/tei_odds.rng - The old reference is only commented-out, do you think we should revert to that reference or keep the one to the online source?