(feat) implemented the ability to pass render_data a stream to write to

Implemented the ability to pass `Template::render_data` a stream to write to. Existing test passes using `MemWriter` and stringifying the bytes:

![image](https://cloud.githubusercontent.com/assets/2935356/4352913/49d0560e-422c-11e4-97dd-994e8cab6a8d.png)
![image](https://cloud.githubusercontent.com/assets/2935356/4352903/37c18050-422c-11e4-9f03-6c5aaa58a1bc.png)