Adding 'invisible' function to Graphite Web

Description of usage

Ran on current version of `synthesize`:

https://localhost:8443/render/?width=941&height=542&_salt=1408374478.348&from=-1hours&fontSize=16&target=invisible(legendValue(integral(collectd.graphite.apache-graphite-web.apache_requests)%2C%22max%22))&target=legendValue(collectd.graphite.apache-graphite-web.apache_requests%2C%22total%22)

And a little more clear:

https://localhost:8443/render/?width=941&height=542&_salt=1408374478.348&from=-1hours&fontSize=16&
  target=invisible(legendValue(integral(collectd.graphite.apache-graphite-web.apache_requests),"max"))
 &target=legendValue(collectd.graphite.apache-graphite-web.apache_requests,"total")
 
 In this case (for testing) the two metrics are the same and result in the same legend values. But with this patch the metrics don't need to be the same at all. One could f.ex. have a graph with latencies, and a legend item stating how many requests were served in that period.
 
 This results in the following graph: http://imgur.com/qSGWmys
 (Mind the Y-axis: it scales down to the one visible metric, not the invisible (much larger, number wise) metric.

functions.py diff

New function:
  
  def invisible(requestContext, seriesList):
  """
  Accepts one or more series, and will remove that from the graph, but will leave it in place in the Legend.
  """
  try:
    seriesList.options['invisible'] = True
  except AttributeError:
    for series in seriesList:
      series.options['invisible'] = True
  return seriesList

+ needs to be added in composer (easy) and at the bottom of functions.py

glyph.py diff

For illustration purposes only (would need to rewrite to do both drawAsInfinite and invisible, of course. But describes the scale of changes needed.

<         sumSeries.append( safeSum( [series[i] for series in self.data if not series.options.get('drawAsInfinite')] ) )
>         sumSeries.append( safeSum( [series[i] for series in self.data if not series.options.get('invisible')] ) )

And:
<       yMaxValue = safeMax( [safeMax(series) for series in self.data if not series.options.get('drawAsInfinite')] )
>       yMaxValue = safeMax( [safeMax(series) for series in self.data if not series.options.get('invisible')] )