Add a streaming usage and test files for this module

see the below code:
```javascript
var transformer = new sshKeyToPEM.Transform();
var source = fs.createReadStream('./test/ssh.key');
var pemfile = fs.createWriteStream('./test/res.pem');
// progress is easy, right?
source.pipe(transformer).pipe(pemfile);
```

This is a better choice like using the official module `gzip`, and I think we can streaming everything without extra encoding and decoding.

You could see http://nodejs.org/api/stream.html#stream_readable_pipe_destination_options for `gzip` usage in pipe.