Unity editor menu item for creating simple class. Place this file into 'Editor' folder.

CreateClassMenuItem.cs

using UnityEditor;
using System.CodeDom;
using System.IO;
using System.CodeDom.Compiler;

public class CreateClassMenuItem {
	[MenuItem("Assets/Create/File/Class...", false, 13)]
	private static void CreateClass() {
		var filePath = EditorUtility.SaveFilePanelInProject("Save file", "Class", "cs", "Enter class name");
		if (!string.IsNullOrEmpty(filePath)) {
			using (var streamWriter = new StreamWriter(File.Create(filePath))) {
				using (var textWriter = new IndentedTextWriter(streamWriter, "    ")) {
					var fileName = Path.GetFileNameWithoutExtension(filePath);
					var unit = BuildClassUnit(fileName);
					var provider = CodeDomProvider.CreateProvider("CSharp");
					provider.GenerateCodeFromCompileUnit(unit, textWriter, new CodeGeneratorOptions());
				}
			}
		}
		AssetDatabase.Refresh();
	}

	private static CodeCompileUnit BuildClassUnit(string className) {
		var unit = new CodeCompileUnit();

		var @namespace = new CodeNamespace();
		@namespace.Imports.AddRange(new [] {
			new CodeNamespaceImport { Namespace = "UnityEngine" },
			new CodeNamespaceImport { Namespace = "System.Collections" }
		});
		unit.Namespaces.Add(@namespace);

		var declaration = new CodeTypeDeclaration {
			Name = className,
			IsClass = true
		};
		@namespace.Types.Add(declaration);

		return unit;
	}
}