Streaming

This is a major release for gatecoin API library, breaking backward compatibility.
Library users have to modify the code to use version 2.

Change log:

• A new streaming functionality.
  It enabling clients to receive messages sent by Gatecoin about different events,
  like trades or tickers. See the readme file for a usage example.
• The library was converted to net standard 2.0
• Nuget packages are available at https://gds.myget.org/feed/public/package/nuget/api-gatecoin-dotnet, starting with this version.
• Build server changed to gitlab

An official release for the .Net 4.5

This is a first official release of the Gatecoin api library for the C#.
It contains the current code and introduces versioning for the project.
At the same time, it is the last major version for a full .Net framework