Fixes #2346

 * Do not display Conversion rate from "All columns" view, when there is "per goal" metric available
 * Do not display conversion rate on the "Goals" view
 * Remove nb_visits_converted from the API when there is "per goal" metric
 * Revenue per visit, is now based on the sum of revenue for all goals, rather than trusting the "revenue" field in the visit (in case Goal Revenue was attributed to a Referrer that is different from the visit Referrer where this conversion occurred)

API Breaking: nb_visits_converted not already in response anymore (only when there is no "per goal" metrics for this report)

git-svn-id: http://dev.piwik.org/svn/trunk@4525 59fd770c-687e-43c8-a1e3-f5a4ff64c105