Nord VPN support

• Issue #5: NordVPN support
• Issue #16: Filter by provider in ProxiesList
• Issue #20: Remove old tests and fix TestProvider
• Issue #17: maxmind geoip2 should be optional
• Issue #22: Get country in Proxynova provider
• Issue #19: Proxynova has changed its script element
• Issue #18: Download geoip2 using maxmind license
• Issue #15: Migrate database schema
• Issue #14: Serve web docs
• Issue #13: Create docs
• Issue #6: Countries list