Fixes #3275, add 'Visits by Day of Week' report to VisitTime as related report for 'Visits per local time'.

Notes:

  * Added ability for GenerateGraphHTML to pass data to GenerateGraphData class.
  * Added ability for reports to show all x-axis ticks on bar graphs if desired.
  * Modified DBStats overview report to show all x-axis ticks on bar graph.
  * Fixed bug in datatable summing where string column values would not get added even if there was no corresponding value in one table.


git-svn-id: http://dev.piwik.org/svn/trunk@6596 59fd770c-687e-43c8-a1e3-f5a4ff64c105