Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform (#1727)

Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform

This also updates the version number to 3.0.0-beta4
diff --git a/csharp/.gitignore b/csharp/.gitignore
index c88f741..8ba8849 100644
--- a/csharp/.gitignore
+++ b/csharp/.gitignore
@@ -1,16 +1,10 @@
-#
-# 	Untracked directories
-#
-src/AddressBook/bin
-src/AddressBook/obj
-src/Google.Protobuf/bin/
-src/Google.Protobuf/obj/
-src/Google.Protobuf.Conformance/bin/
-src/Google.Protobuf.Conformance/obj/
-src/Google.Protobuf.Test/bin/
-src/Google.Protobuf.Test/obj/
-src/Google.Protobuf.JsonDump/bin/
-src/Google.Protobuf.JsonDump/obj/
+# Output
+bin
+obj
+project.lock.json
+TestResult.xml
+
+# Possibly legacy now?
 mono/bin
 mono/tmp
 mono/protoc
@@ -23,6 +17,7 @@
 #
 # 	Untracked files
 #
+.vs
 *.user
 *.suo
 *.nupkg