Ability to respond with a binary content

Hi,

##### Feature:
- responder has the ability to return stream content to the client

##### Notes
- may be it will be a good idea to write an example to show how to handle it on client-side. However, such example is in responder.functional.js
- lint throws an issue on test/run.js because optimist uses "default" keyword. I found that we can re write this part of code the following way

Before
```javascript
.usage()
.default()
``` 
After
```javascript
.usage()['default']()
```
but it looks a little bit ugly

--
Best Regards,
Alexey