Add shopify as a sponsor in footer

Closes #2128 

* Make an SVG of Shopify's logo in 555 gray color
* Add Shopify in sponsor section of footer
* Slightly resize footer sections to prevent overflow onto second line
* Reword "Getting" to "Get" to prevent overflow onto second line

This doesn't exactly match anything that Shopify explicitly approves in:

http://www.shopify.com/press/design-guidelines

Should I reach out to them and ask if it's OK? Or can we just deploy as
is?

Is SVG graphic OK?  Or should I change it to PNG like the other logos?