Gif timeserie

Pull request related to http://www.mapserver.org/development/rfc/ms-rfc-111.html

This pull request adds a new format to MapCache, Gif. It also has a new write_frames function to encode an animated image. 

It includes new attributes for the time dimension, animate and delay.

The check to see if we need to animate is in mapcache_core_get_map. It checks the assembly_type of the Tileset.