support for serving .gz version of a file (autogz=true)

Please accept this commit which adds support for automated serving of a `.gz` version of a file. The gzipped version of a file will automatically be served instead of the original file (without the .gz extension) if all of the following conditions are met:
- Passing `autogz: true` options.
- GZip is supported by a client.
- GZipped version of a file exists.

### Example
Directory:
```
main.css
main.css.gz
```
Code:
```js
app.use(function *(){
  yield send(this, this.path, { 
    root: __dirname + '/public', 
    autogz: true 
  });
})
```
URL call:
```
/main.css
```