Add function to get the longest single session. Update readme.

Fixed a bug where sessions starting one day and running through midnight to the next day would be backwards. Still won't work for sessions more than 24h in length but it is a good holdover until proper dates are parsed too.

Added the ability to pass the log directory into session.fromdir(dir) to parse all log files. This auto concatenates logs across files, so it can work on sessions spanning multiple log files.

Made the "FromFORMAT" methods static members of the session class