Introducing documented piwik.php PHP Tracking client.

Also adding integration test suite in place.

 * Adding new mechanism to trigger known tracking requests, then call all API methods get* and compare XML to the previous 'expected' XML.
 * make sure overwritten config files are not read during tests. Use defaults from global.ini.php.
 * piwik.php Tracker allow calling script to force Datetime and user IP for a specific request. 
 * Wrote 3 main tests
  * wrong tracking requests should not return error and should not record anything 
  * standard one visitor, 2 visits, with page view, download, outlink and goal tracked
  * Added integration test as an example in the ExampleAPI plugin (pretty simple to add testing code)
TODO 
 * Finish PiwikTracker and show it in UI
 * Show how to use image based tracker in UI
 * Add more tests (multi periods and multi sites) in Main.test.php
 * Fix failing Config.test.php test (when ran via all_tests.php doesn't fail otherwise)

git-svn-id: http://dev.piwik.org/svn/trunk@2429 59fd770c-687e-43c8-a1e3-f5a4ff64c105