Allowing log file location and log level to be set via env configuration

Oh hey there, 

This was something we needed to be able to deploy jennifer, figured it might be helpful for others, as the directory that we pull the code to isn't writable by the user that runs the process. 

Just added LOG_LEVEL and LOG_FILE environment variables to make them configurable, there are also some defaults as well, so if you don't set those env vars jennifer still behaves in the same way she always has.