Fixes #2670

 * Metadata output now contains "imageGraphUrl"
 * the function used by piwik mobile and others, API.getReportMetadata now accepts period and date parameters
  * for "Evolution graphs" (goals, visitssummary, etc.) we must "plot" multiple days previous to the currently selected date. 
  The metadata APi automatically does the $date parameter rewrite, when period != range, and date is not already a range.
  Therefore the imageGraphUrl can be used as is.
 * New page available at: index.php?module=ImageGraph&action=index&idSite=1&period=month&date=2011-04-17
  * This page will call metadata APi then display the graph for all existing reports. This is a useful debug/test page to view all graphs as they will be displayed in Piwik Mobile (and PDF reports later refs #2706

Refs #1721
 * Showing less vertical bars in graph by default 
 * Displaying empty graph when there is no data

git-svn-id: http://dev.piwik.org/svn/trunk@5296 59fd770c-687e-43c8-a1e3-f5a4ff64c105