v3.1 Release with more stable client-server protocol

Fixed: headers problem if flush() or ob_end_clean() is called before script shutdown
Fixed: problem with frameworks that overrides $_SESSION handler see https://github.com/barbushin/php-console#troubleshooting-with-_session-handler-overridden-in-some-frameworks
Migration: Update PHP Console extension to last version >= 3.0.20

[Fixes #34]
[Relative #36]