Accessibility/Validation fixes

Updated index.html to fix a handful of minor accessibility and validation issues.
- Most images had title tags instead of alt tags
- There were a few unencoded & floating around in the Xero URLs
- <div class="hr"/> wasn't playing nice with the HTML minifier
- The mobile menu was missing a </ul>
Merge pull request #1 from SpoonNZ/master

Accessibility/Validation fixes