Added img alt attribute with author's name

While validating the HTML on my Fuel CMS installation, the Validate tool pointed out that there was an image tag missing an alt attribute on ```/blog/authors``` and ```/blog/authors/1```.

I updated the default theme's views with alt attributes by passing ```'alt' => $author->name``` to the ```blog_users_model``` function ```get_avatar_img_tag()```.