Added 'dynamic' columns mode where number of columns varies dynamically to fill the width of the gallery.

Proposed addition to allow galleries to dynamically fill the width instead of having a fixed number of columns.

Implemented as a new 'dynamic' option in the 'number of columns' setting. This setting causes the table div and table row divs to not be generated and for a different css class "afg-item" to be used for each gallery item, instead of "afg-cell".

Is compatible with all selectable photo sizes, including custom sizes.