Done.
Fixing tests.
Refs #4200, documented Date object.
Refs #4200, documented Db.php and small refactor to LogDataPurger/ReportsPurger (allow order by to be unspecified).
Refs #4200, documented the Filesystem class.
Refs #4200, finished documenting FrontController.
Refs #4200, documented Http class.
Refs #4200, tweaked docs for core/IP.php.
Refs #4200, added more documentation to the Log class and modified default value of log_file_path config option.
Refs #4200, documented Mail class.
Refs #4200, documented several more classes.
Refs #4200, finish documenting core/Piwik.php.
Documented core/Plugin.php class.
Refs #4200, tweaked RankingQuery docs.
Refs #4200, documented core/Segment.php
Documented core/SettingsPiwik.php.
Refs #4200, documented core/SettingsServer.php.
Refs #4200, documented core/Singleton.php and core/Site.php and set method visibility for Site.
Refs #4200, documented task scheduler class.
Refs #4200, revert getAllTableMetadata function removal and document core/Url.php.