[Review] Migrate functionality from Publisher

### Problem

Currently the rendering functionality of **Alephant** sits within the [Publisher](https://github.com/BBC-News/alephant-publisher) (moved in [#5](https://github.com/BBC-News/alephant-publisher/pull/5)). Due to a new publisher gem being created to deal with messages that are received via a request, the functionality needs to be extracted back out into this gem. **Note**: this gem is not currently being used anywhere.

### Solution

Move the current rendering functionality into this gem, ignoring what is currently in this repo. After this is done the **Publisher** will be refactored to use this gem.

![green_lantern_lol.png](http://media.giphy.com/media/KvKcxE90OX7Ta/giphy.gif)
I'm pretty sure that `subject` is automatically set to an instance of `Alephant::Renderer` implicitly as it's referenced in the describe above.