Improvements and bug fixes.

[VashurkinAnton]

The following bugs are fixed:

• The this.Agent will never reseted since the object this.DefaultAgent is used by reference;
• The iOS 13 iPad detection on browser side: iOS 13 iPad user-agent #129.

The improvements are made:

• Structure of constructor changed to prototypal inheritance - which makes easy extensions and usage more safety;
• Now the user agent and request headers can be passed while the instance creation, and it will be automatically parsed and saved to instance;
• Moved out the regex's from instance data to global module space for save the memory (that are critical since can save big amount of memory within express.js usage);
• Added the inline jsdoc documentaion;
• Added support of browser detection by default;
• Added the webview detection (for browser side on iOS and everywhere for Android);
• The .parse method moved to static UserAgent.parse (the instance.parse still supported but will emit deprecation message on call);
• The .testNginxGeoIP accessable as static method too;
• Removed the constructor call from the end of file (never used and just triggers the garbage collector while first initialization);
• Express middleware now the same function which optimize the memory usage without any side effects;