Image column database url source fix, optional image width/height

I made some fixes/enhancements on the Columns\Image class.
First, the class was not able to display image if the image url was a field in the database. 
Now both Gravatar (and basically DataPopulation based images) and images based on database field value work.

Second, I added the possibility to set exact width/height for the image rendered.
One can simply set image size this way:
```php
$profileImageColumn = new Column\Image('avatarUrl', 't');
$profileImageColumn->setLabel('Avatar');
$profileImageColumn->setImageWidth(100)->setImageHeight(100);
```