Add support for braces

[frmatthew]

See http://gregorio-project.github.io/gabc/details.html#braces

Will require:

• improvements to the gabc parser to handle instructions in []
• a new brace visualizer type to draw the brace (maybe a couple different types depending on the brace type?)
• also, at least one new marking type to span the brace over notes

How important is it to support millimeter instructions in braces? Manual size specs seem pretty useless for web based rendering...

[frmatthew]

Fixed by 896731dea6d75cebd58391b2ebf74e460e5ddfc9