Exit rate should use visits to the page, not hits #2069

Reported in http://forum.piwik.org/read.php?3,71745

(In [3870]) Refs #1736

• API functions returning data now have a new optional 'segment' parameter. segment can define a Visitor segment dynamically that will be applied to the report. For example, &segment=country==FR;actions>=3 (AND, OR supported. Only == and != supported currently, but easy to add more)
• For API requests with a segment parameter, the reports will now be processed on the fly, and only the requested plugin report will be archived.
• All plugins now define the 'segments', with a name, category, SQL field, filter, etc.
• Simplifying archiving code a bit
• Fixes #2069 Exit rate computation
• New widget: lists the Top Keywords for a page URL, Widgets for a website only. Maybe later we could create a widget category "For your site"?
  • This widget is pretty cool SEO wise, but maybe the PHP snippet should do caching (not so good hitting the API on each page view... but why not?)
• still to do!