DOC : edits to optional dependencies

 - cleaned up failed merge
 - added external movie programs to docs (closes #2490)
 - added all the external libraries that are include in the source
   tree
 - added six as explicit dependency.  This was discussed before and
   rejected, but I added it here because we now have a minimum version
   check which should be documented.
Merge pull request #2950 from tacaswell/install_dep_doc

DOC : edits to optional dependencies