Logging part completely rewrited

Hello, I rewrited the logging part of lshell to allow the user to configure different handlers (mail,syslog,file,stderr,etc). I also changed logging level of some commands/output.
Lshell will now be able to truly report incidents like kicking or forbidden commands.
I've updated the doc (man+readme) but it will maybe need some reviews as it's clear for me but can be complicated for people who don't know logging module and his configuration.