Fixed IDisposable implementation and usage.

- Implement IDisposable interface correctly, with sealed classes where possible for simplicity.
- Add using statement around undisposed local variables.

OpenRA.Game/InstallUtils.cs

@@ -93,8 +93,9 @@ namespace OpenRA
 			List<string> extracted = new List<string>();
 			try
 			{
-				var z = new ZipInputStream(File.OpenRead(zipFile));
-				z.ExtractZip(dest, extracted, s => onProgress("Extracting " + s));
+				using (var stream = File.OpenRead(zipFile))
+				using (var z = new ZipInputStream(stream))
+					z.ExtractZip(dest, extracted, s => onProgress("Extracting " + s));
 			}
 			catch (SharpZipBaseException)
 			{