fix 'Invalid cookie token' message when no cookie is set
make home dirs private in sudo example

tests use case where sudo failed to set cwd
catch os.chdir errors

when spawning subprocesses.
Walk up until something works,
eventually falling back on a temper.
ignore some more files in .dockerignore
make Spawner.notebook_dir configurable

resolved based on user's home dir
Merge pull request #91 from minrk/sudo-cwd

handle some failures in sudo