gnt-job list: deal with non-ascii encoding in jobs

This fixes issue 432. If a job summary contained a non-ascii character,
gnt-job list crashed badly so far. This patch fixes the crash and makes
gnt-job list display unicode characters properly.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>