feat(directive): translate attributes using directive

translate directive can process attributes now. as per #568 
If we use ```$translate(translationIdsAsArray, scope.interpolateParams, translateInterpolation)``` we just use this loop [src/service/translate.js#L731](https://github.com/angular-translate/angular-translate/blob/master/src/service/translate.js#L731) to finally call ```$translate``` with a non-array value here [src/service/translate.js#L728](https://github.com/angular-translate/angular-translate/blob/master/src/service/translate.js#L728). I don't know if it is more efficient but the ```$translate``` in its array version does not behave as intended (the results object populated here [src/service/translate.js#L724](https://github.com/angular-translate/angular-translate/blob/master/src/service/translate.js#L724) should provide information about success or failure of the translation.