Add html5shiv to wp_head hook for older browsers

See #51
Rename thematic_html5shiv filter to thematic_html5shiv_output for clarity
Add inline hook documentation to thematic_add_html5shiv()
Update docbloc of thematic_add_html5shiv()
Merge pull request #68 from ThematicTheme/add-html5-shiv

Add html5shiv to wp_head hook for older browsers
See #51