Release v1.4.0

• Performance improvements, send ~10.000 notifications/minute in a 2 GB RAM/1vCPU server. Related: #81 and #86
• Parallel flushing size parameter was added with a default value of 50 notifications.
• Custom welcome message that can be enabled/disabled. Thanks to @mociofiletto. Related #91
• Added the 'perfecty-push' suffix to the server_url option.