Implement friends section
Merge pull request #34 from asciidisco/friends

Implement friends section
Removed note about NOT using build tools
For @anselmh: Use ndash instead of mdash
Changed BT logo
Twitter card meta tag

Added Twitter card meta tag to preview of site in tweets. Without the
meta tag, it’s not working. I chose product because it was the best fit
for our logo. More card types:
https://dev.twitter.com/docs/cards/validation/validator
Doubled social meta tags

Removed doubled social meta tags. Must have been a merge error. Now
corrected.
Added Qwaiko to the list of sponsors
Added Aral Balkan to the list of speakers
Added Aral Balkan to the list of speakers
Added trivago to the sponsors
Set z-index on all elements in header, remove z-index: -1 on background

This prevents a douplicate scrollbar on Chrome.

Closes #24.
Closes #35.