Generic search engine detection

[robocoder]

To reduce ongoing maintenance and set some acceptance criteria for new additions. At the same time, potentially improve attribution stats.

If the referrer contains a well-known keyword parameter (eg 'q', 'query'), automatically treat as a searchengine. (Be careful here...too many recognized parameters increases the chances of false matches). The domain name will serve as the search engine name. A generic icon will be used. No link to the search engine will be presented (to deter referrer spam / phishing attack).

Instead, if the number of hits exceeds a threshhold (which varies depending on the period or date range), the link will submit the candidate search engine domain to a piwik service for future inclusion in SearchEngines.php.

Changes to core, new plugin, and piwik.org service api.