Alt tags for images

Refinerycms-images will now accept title and alt attributes for images.
If not supplied both will default to the image file name
Title and alt are available in the index views (grid_ and list_ ) on hover over the thumbnail or title respectively.
For this you can use:

```ruby
image_title.presence || CGI::unescape(image_name.to_s).gsub(/\.\w+$/, '').titleize
```