Fixes #37

Fixes issue: Nesting groups without index pages could lead to
'Collection was modified' exception.

src/DocNet/NavigationLevel.cs

@@ -90,9 +90,12 @@ namespace Docnet
 		public override void GenerateOutput(Config activeConfig, NavigatedPath activePath)
 		{
 			activePath.Push(this);
-			foreach(var element in this.Value)
+			int i = 0;
+			while(i<this.Value.Count)
 			{
+				var element = this.Value[i];
 				element.GenerateOutput(activeConfig, activePath);
+				i++;
 			}
 			activePath.Pop();
 		}

src/DocNet/Properties/AssemblyInfo.cs

@@ -32,5 +32,5 @@ using System.Runtime.InteropServices;
 // You can specify all the values or you can default the Build and Revision Numbers 
 // by using the '*' as shown below:
 // [assembly: AssemblyVersion("1.0.*")]
-[assembly: AssemblyVersion("0.13.0.0")]
-[assembly: AssemblyFileVersion("0.13.0")]
+[assembly: AssemblyVersion("0.13.1.0")]
+[assembly: AssemblyFileVersion("0.13.1")]