provide default plus_cursor to passed-in elements

This patch embeds the plus_cursor.png image from Intense Images' demo folder
into Intense Images itself.  This is similar to how Intense Images already
embeds a default close_cursor.png image in the viewer.

A user can override this default cursor by specifying !important in CSS:

    img {
     cursor: crosshair !important
   }

Similar to how a user can also override the default close cursor in CSS:

    figure > img {
     cursor: crosshair !important
   }