StaveNote and Stem styling

#### Description:
This pull request extends the current styling options by allowing styling of stems (previously only noteheads & modifiers were supported) and adding an ease-of-use function to allow the styling of a complete Vex.Flow.StaveNote with one call.

#### Summary of Changes:
Added setStyle() class to stavenote.js, to set a style for each key and to the stem.
Added three styling classes to stem.js:
- setStyle() -- set a style for the stem
- getStyle() -- return the stem's style
- applyStyle() -- update the rendering context with the style for drawing

Added drawNoteStyles() test to stavenote_tests.js