Fixes for *_id_type > int

These patches modify the packed_range code to handle cases where we
don't want to pack everything into 32-bit integers.  This should fix
some runtime bugs in the 8-byte dof_id_type case.
Default to 32-bit XdrIO for now
unsigned int -> dof_id_type fix
Merge pull request #111 from libMesh/bigids_fixes

Fixes for *_id_type > int