Implement caching in full

[ronkorving]

That means:

• turn off all caching for non-GET/HEAD (apply http://richhewlett.com/2015/02/16/preventing-browser-caching-using-http-headers/)
• enable caching using last-modified timestamps (one for a resource, and the most recent one also at collection-level)

Update: this has been implemented at the collection level, not yet at resource level.