null values should be preserved as a property
remove unused code paths dealing with defaults
Merge pull request #1302 from ContinuumIO/bugfix/1298

null values should be preserved as a property
Rename Glyph -> GlyphRenderer and BaseGlyph -> Glyph
Update examples after renaming glyphs
Merge pull request #1310 from ContinuumIO/mattpap/rename_glyphs

Rename Glyph-> GlyphRenderer and BaseGlyph -> Glyph
merge master