Improve campaigns tracking: URL builder, JS API and user documentation #1042

• See new Tracking Campaigns
• URL Builder tool
• Official piwik campaign parameters names are now: pk_campaign and pkkwd (consistent with cookie prefix, better than piwik prefix)
• New FAQ: "Does Piwik support Google Analytics campaign parameters (utm_campaign, utm_medium, utm_source, utm_term)?"
• New FAQ: "How do I customize the Piwik Campaign parameters pk_campaign and pk_kwd?"

(In [4504]) refs #1042, fixes #2329 - added setCampaignNameKey() and setCampaignKeywordKey() methods to tracking API; going to update the online docs now

See also #2329, requires FAQ

(In [4571]) Fixes #604, #1042

• See new Tracking Campaigns doc at: http://piwik.org/docs/tracking-campaigns/
• URL Builder tool: http://piwik.org/docs/tracking-campaigns/url-builder/
• Adds links to the tracking campaigns & URL builder directly on the Campaigns report in Piwik.
• Official piwik campaign parameters names are now: pk_campaign and pkkwd (consistent with cookie prefix, better than piwik prefix)
• New FAQ: "Does Piwik support Google Analytics campaign parameters (utm_campaign, utm_medium, utm_source, utm_term)?" http://piwik.org/faq/general/#faq_119
• New FAQ: "How do I customize the Piwik Campaign parameters pk_campaign and pk_kwd?" http://piwik.org/faq/how-to/#faq_120