Resolve "Setup logging directory with unprivileged user access"

Closes #16 (closed)

This adds a --logdir argument. If provided, logs are written in the /var/log/ocspd directory (to ocspd.log) and traces for unexpected exceptions are placed there as well. Optionally, one can change the default log directory by adding an argument to --logdir. If not provided, no logs are saved to file, traces are saved to /var/log/ocspd, which has to be created during installation (part of #26 (closed) afaik)