Add flushCompression as an alias for flush

Supports use such as:

```javascript
// Not going to write again for a while.  If compression middleware is
// installed let's tell it to flush what we've got through to the client.
if (res.flushCompression) {
    res.flushCompression();
}
```

Currently the `res` object has a `flush` method regardless of whether
compression middleware is installed.  The built-in `flush` method issues a
noisy deprecation warning.  This patch provides an alias for the `flush`
method to allow detection of the compression middleware's `flush`.

This addresses expressjs#72.

index.js

@@ -66,7 +66,7 @@ function compression (options) {
     var _write = res.write
 
     // flush
-    res.flush = function flush () {
+    res.flush = res.flushCompression = function flush() {
       if (stream) {
         stream.flush()
       }