Improved the Cache class
Make use of new Cache 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Removed outdated function get_cached()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
Changed include order in bootstrap
Moved system startup to bootstrap
Make use of ww_config() in bootstrap
Removed constant WW_ROOT_URL
Added more error handling
Replaced unnecessary Exception triggers with error triggers
Added method User::exists()
Revised the session system
Added method hasRows() to Database_..._Result classes
Added method UserGroup::exists()
Revised the User class
Revised the UserGroup class
Restructured the settings.ini file
User class: Session online threshold now defined in settings