Tests and implementation of logging.
Fix broken logic branch involving existing directories.
Remove default stderr observer.
On second thought, stderr output is somewhat useful to humans.
Lint fixes.
Merge remote-tracking branch 'origin/master' into minimal-logging-416
Always restore the original euid
Random unrelated fix: link to install docs.
Document where logs go.
Merge branch 'minimal-logging-415' of github.com:ClusterHQ/flocker into minimal-logging-415

Conflicts:
	docs/usage.rst
Fix typo.
Merge pull request #420 from ClusterHQ/minimal-logging-415

Minimal logging.

Fixes #415.
Fixes #408.