Upgraded library to NetStandard 2.0

Changelog:

• Added NetStandard 2.0 support
• Renamed the compiled library name
• Updated RestSharp, Newtonsoft.Json and Nunit
• Updated the Nunit tests for the new syntax (thanks to @nicholi and his work)
• Updated several models to have the new fields
• Refactored the README file to increase legibility
• Updated README file with the visitor's endpoint usage instructions

Bugfixes:

• #96 rename library output dll
• #90 moved to netstandard 2.0
• #85 company tags now are available when viewing a company