Add Percussion Sample to Music21 Corpus Demo Section

[kir12]

Motivation
Currently (as established in this music21 Google Groups post) there aren't any scores inside the music21 corpus that have Percussion parts, since all the pieces in the corpus (as well as probably all eligible pieces) are old enough that Percussion/Drumset parts hadn't really existed yet. Having at least one reference for what a Percussion part in MusicXML should look like for maximum compatibility with music21 would be great for verifying the correctness of our own MusicXML files. (As well as pre-emptively ruling out music21 as the source of any problems)

Feature summary
The aim of this feature is to add this MusicXML sample coming directly from the W3C Community Group (which created the MusicXML spec) to the music21 corpus. It's likely the most authoritative a MusicXML example for Percussion parts can get, and the MusicXML Specification's license seems to allow usages such as this.

Proposed implementation
A file titled drum_sample.xml will be placed in music21/corpus/demos/drum_sample.xml which will have the contents of the MusicXML example.

Intent

[x] I plan on implementing this myself.
[ ] I am willing to pay to have this feature added.
[ ] I am starting a discussion with the hope that community members will volunteer their time to create this. I understand that individuals work on features of interest to them and that this feature may never be implemented.

[mscuthbert]

Good issue. Glad to take a commit.