added run method to execute yii commands

Discussion: https://github.com/yiisoft/yii2/issues/350

ToDos:
* add params
* execute by namespace
* autoload by namespace

Can be tested with this `composer.json` and `composer install`:

	"scripts": {
		"post-install-cmd": [
			"yii\\composer\\InstallHandler::setPermissions",
			"yii\\composer\\InstallHandler::run"
		],
		"post-update-cmd": [
			"yii\\composer\\InstallHandler::setPermissions"
		]
	},
	"extra": {
		"run": [
			["help"],
			["hello", "Hello World!"],
			["yii\\console\\controllers\\HelpController"]
		],
		"writable": [
			"runtime",
			"www/assets"
		],
		"executable": [
			"yii"
		]
	}

Will execute `help` and `hello` (without params) without errors, but currently fails on yii\\console\\controllers\\HelpController.