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.

cc @KushalP @dcarley @tombooth @nosborn