* removed useless spells.spell column that was duplicate of spells.words
* Usage_Statistics reports now plugin version too
* (internal) removed $hook_types array, using defined() and constant() functions now
* (internal) optimized Spells class
* can be used to install plugins from command line. Usage: "php install_plugin.php path_to_file"
* fixed if HTTP_HOST not set in common.php when executed from command line
* Added 'Are you sure?' popup when uninstalling plugin
* Moved plugin install logic to a new class: Plugins
* added some warnings when plugin json file is incomplete