Add fonts

[muffinresearch]

• Add Fira Sans webfonts (woff2, woff and otf) we don't need eot since we won't be worrying older IE versions.
• Fix clean script to remove everything from dist except the .gitkeep file.

[kumar303]

I see some binary files that appear to be unused in the scss, like UltraLightItalic and ExtraBold, etc. Do we still need to include them?

[kumar303]

This looks similar to what's in the karma config. Is there an easy way to combine them?

[muffinresearch]

Yep, that's a separate yak to shave issue is filed here: https://github.com/mozilla/addons-frontend/issues/353

[kumar303]

I don't know much about fonts but r+wc :)

[muffinresearch]

I see some binary files that appear to be unused in the scss, like UltraLightItalic and ExtraBold, etc. Do we still need to include them?

I'll leave the extra fonts for now, it's early days so we don't really know what we might need. Also there's a possibility of using an entirely different font, in which case all this will be replaced.

However, I'm making mixins in another branch to make any replacement of font-families painless.