Implementing Site Search tracking & reporting in Piwik core!

 * New Admin UI to customize, for each site, wheter site search is enabled. Also options to set default values to use.
 * New Reports: Searches, Searches with no result, Search categories, Top Pages Following a Search
 * to track "No result keyword" users will have to tag their site with a JS call, or add a new parameter to the search result page &search_count=X (X being zero for no result searches)
 * Reports works with Row evolution, PDF/HTML reports, Piwik Mobile
 * idaction_url is now NULLable because, Site Search records a page with idaction_name == Keyword, and idaction_url == NULL. This ensures that the Site Searches don't create "Page URL Not defined" records. 
 * updates to Tracker JS API, new function trackSiteSearch, also added in PHP tracker
 * New fields in log_visit to track searches
 * new segment, "searches" which can be used to select visitors who did a search ie. searches>0 or those who searched a lot, ie searches>10
 
 TODO: 
  * commmit integration test, TESTING, DOCS, FAQ, release, and a nice Prayer to the universe and the stars, hoping that I can code a major new feature without any bug...
  * It would be awesome to have compatiblity with Transitions so we can see, for a given site search, what are the starting pages and Destination pages
  
 Thank you for your patience Timo, 
 and thank in advance everyone for your help Testing this new feature!
 


git-svn-id: http://dev.piwik.org/svn/trunk@7190 59fd770c-687e-43c8-a1e3-f5a4ff64c105