Use DejaVu Sans Book as a fallback for bold/italics

DejaVu Sans Book has 5942 glyphs while bold and italic have 5868 and 5155 glyphs.

They may not quite match, but unifont doesn't have bold or italic glyphs at all so it's still closer.
Merge pull request #309 from pnorman/dejavu_fallback

Use DejaVu Sans Book as a fallback for bold/italics