Changes to run mongosback without root right

Add PID_FILE location in configuration, allowing to set a directory where a simple user has right to write. Ex : "/home/pascoual/tmp".
For the ability to write on /var/log/mongo-backup.log, simple unix commands do the trick : 
# sudo touch /var/log/mongo-backup.log
# sudo chown pascoual /var/log/mongo-backup.log

After that you can run mongosback with simple user.