Revised the settings system

* Settings now reside in a special INI file (site dir)
* Added an advanced INI parser that decodes JSON values
* Renamed settings section "core" to "main"
Improved the System class

* Added getters for settings and database driver
* Made the class failsafe
Added function ww_setting()
Make use of function ww_setting()
Revised the Database class
Converted Database::loadDriver() to function ww_open_db()
Added new caching implementation (class Cache)
Improved the Cache class

Added ability to use multiple cache files within one instance
Make use of new Session class
Removed outdated function get_cached()
Improved method System::loadModule()

* Added constants WW_MODULE and WW_MODULE_PATH
* Stop if a module has already been loaded
Autoloader detects module path automatically
Reordered the directory structure