scanner throws "INVALID_INVOCATION_PARAM_NOT_STRING" for valid invocations

@medikoo we need to rethink one matter. Right now scanner will protest if we scan such code (because we require a string literal at the moment):

```javascript
_('string to be translated but due it\'s impressive length' +
' it needs to be broken into more than one line...')
```

I propose to remove checking for param being string literal and just go to evaluation. What do you suggest?