Made CellArray.h public for extension authors to use.

Note: Works ALOT better then Vectors...
About the note, well, in http://github.com/WildCard65/FF2-Alpha I'm making an extension that used an IBasicTrie + CUtlVector combo... but it ran into an issue where the vector f***ed up somewhere in it's storing system.  
I solved the issue by replacing all usage of CUtlVector with CellArray, so I believe this should become public.