refs #6577 Return HTTP 204 instead of GIF for JavaScript tracking.

added a new url parameter send_image=0 to disable sending an
image and instead response with a 204 code
refs #6577 updated changelog
use HTTP 1.1
refs #6577 update minified piwik.js
Merge branch '6577' of github.com:piwik/piwik into 6577
Merge pull request #6659 from piwik/6577

 Return HTTP 204 instead of GIF for JavaScript tracking.