Improvement of src management

When I tried to use the grunt-response-images task, I encounter few errors when I tried to configure the `src` like that:

```javascript
{
  src: ['some/path/**/*']
}
```
This is the error I received:

```bash
Warning: Command failed: gm identify: Unable to open file (some/path/subdirectory) [No such file or directory].
gm identify: Request did not return an image.
 Use --force to continue.
```
That's quite normal. In fact, the task try to give a directory to the image library for resizing. Logically, it is not possible to handle a directory for resizing.

I spent few hours to understand my mistake that it is mandatory to give the `src` like that:

```javascript
{
  src: ['some/path/**/*{jpg,gif,png}']
}
```

In my opinion, we should be able to not be forced to specify the file extensions. It will be more in the way that grunt handle the files.

To solve the issue, I added a check if the the current `src` path evaluated is a directory or not.

I understand that in some cases, when we mix images and other file types in a directory tree, to have the extensions in the `src` is clearly a better solution but in case where we have only images, a simple notation is preferable.