Preliminary hummingbird glyphs added to vexflow. See commit comments

...hummingbird.js) and combined with the original Gonville (src/fonts/combined.js). Actual tests need to be performed to fine tune the Hummingbird glyph "point-size" to be consistent with that of Gonville. A new "test" file (src/hmb.html) provided to check the results visually.