Added support for netstandard 2.0 and 2.1

New build process (GeoJSON-Net#140)

* Experimenting with .net 3.5 framework location for DevOps build

* Adding new build icon to ReadMe for moving build to DevOps

Update GeoJSON.Net.csproj

GeoJSON-Net#123

- coordinates are not validated against geographic coordinate bounds,…

… can now be any numeric

- assembly file version matching nuget package version

Appveyor badge

Removing validation that was put in for lat/long against the specs

fixing tests

Fixing issue 94, exception when checking for null

Changing nuspec file

Linestring to use IPosition in Is_Closed