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.