Re-add --debug option (reverts #11)

The --debug option is somewhat badly named -- it *both* adds debug
output, *and* causes carbon-cache to run in the foreground. Removing the
option in #11 caused the upstart script to lose track of the process as
carbon-cache started unexpectedly daemonizing.

Ideally we want to have a way of running through upstart without the
debug output, but this will fix the immediate problem.
Merge pull request #13 from gds-operations/run-carbon-cache-in-foreground

Re-add --debug option (reverts #11)