Add gallery page

[olgabot]

Glancing at the docs, it's hard to get a broad sense of what color schemes are available without hunting around. It would be helpful to have a gallery of color schemes (and possibly the tumblrs from where they came from!) on the landing page.

[jiffyclub]

If you drill into the docs on specific submodules you can preview all the pre-built palettes, for example http://jiffyclub.github.io/palettable/mycarta/. These are all linked off of the front page and all of them link back to their original sources.

There are ~700 pre-built palettes in palettable so putting those all on a page seems impractical. Many are the same palette in different numbers of colors so if you only went with one per palette you'd get less, about 100 images. I wouldn't put that on the landing page, but it could probably all fit on one page somewhere.

Or are you thinking more of a curated gallery with only a handful of hopefully representative example palettes and more descriptive info about the sources?