Use wave underlines for Fly(make|check|spell) if possible

Wave underlines are available as of Emacs 24.3.  These underlines are
much less intrusive than foreground faces, and thus better suited to
highlight “background” errors from on-the-fly syntax or spell checking.
Using such underlines also makes this theme adhere to the standard user
interface of spell checkers or syntax checkers in other editors or IDEs.
Merge pull request #63 from lunaryorn/use-underlines

Use wave underlines