Version 1.7.1: Unit tests, Default Middleware Extensions

[jdanylko]

Thanks to @mo-esmp for these updates to the library. They include the following:

• Introduced ServiceCollectionExtensions for adding Tuxboard Dashboard services to IServiceCollection: The Middleware extensions are used for when a DbContext (TuxDbContext or other inherited DbContext) is located in the same project. If the DbContext is located outside the main project, use the "long form" of setting up the dashboard (i.e. 10-Default-Dashboards in Tuxboard.Examples).
• Updated TuxboardConfig to implement ITuxboardConfig with XML docs
• Modified Tuxboard.Core.csproj to generate a documentation file and include a framework reference to Microsoft.AspNetCore.App.
• Refactored Tuxboard.Core.nuspec to streamline metadata and dependencies, removed legacy Microsoft.AspNetCore.Mvc.ViewFeatures and include framework references for different .NET versions.
• Add Tuxboard.Core.Tests project
• Added ServiceCollectionExtensionsTests to test AddTuxboardDashboard methods