New glyph etc

here are a couple of minor additions:

- a longa rest glyph, see pull request #36 (+ TC)
- rehearsal mark support (+ TC)
- support for articulations on chords (+ TC)

I also changed the alphabetic duration codes in `tables.js` (q, w, b, l) to numbers. This fixes one issue: When the duration of a chord is not specified in the chord element, but in one of the note children, the chord duration gets calculated by converting the durations to numbers and finding the smallest value. Obviously, this doesn't work when letters are used, too. In addition to 1 (whole note) and 2 (half note), I used the numbering scheme proposed by @zolaemil: 0.5 for the breve, 0.25 for the longa