Send appropriate cache headers

[edtjones]

At the moment the api doesn't deliver any cache headers. We need to fix that so interstitial caches do the right thing.

A new plugin needs to handle:

• etags for entities (and collections??)
• cache-control
• last-modified

This project (GPL3) has some code we can reuse, I expect: https://github.com/gnotaras/wordpress-add-headers

[phendrick]

Created a https://github.com/rooftopcms/rooftop-response-headers plugin