Update logo to 'Teaching'
Merge pull request #141 from github/teach-to-teaching

Update page header logo to "GitHub Teaching"
fly by pull request class notes and resources
Merge remote-tracking branch 'origin/gh-pages' into gh-pages
Merge branch 'gh-pages' into slide-theme-branding
Merge remote-tracking branch 'origin/slide-theme-branding' into slide-theme-branding
Merge remote-tracking branch 'origin/slide-theme-branding' into slide-theme-branding