use POST to submit variables to graphite when requesting data

* the "split up in multiple requests" thing works, but only if
  individual targets were small enough to fit in individual
  requests.  With very long targets we would still get
  'uri too long' errors.  Using POST is a better solution, and
  means we can simplify the code again
* however POST means we can't do jsonp anymore, so we use plain
  json. which is not that bad, since jquery was leaking with
  jsonp anyway.  but requires CORS setup in apache.
* img requests can't be done over POST so for those we just
  don't include targets if they cause the url to become too long