When STATIC_ROOT wasn't set, collectstatic --clear would delete
every files within the current directory and its descendants.
This patch makes the following changes:
Prevent collectstatic from running if STATIC_ROOT isn't set.
Fixed an issue that prevented collectstatic from displaying the
destination directory.
Changed the warning header to notify when the command is run
in dry-run mode.
Loic Bistuer
Jon Lønne
Claude Paroz
Tim Graham