[Review] Update README

Update the README inline with recent changes ([v2.0](https://github.com/BBC-News/alephant-broker/commits/master)).

#### To-do

- [x] Add examples
- [x] Test examples
- [x] Add high level explanation.
- [ ] Diagrams?
- [x] Add `ruby` tag to code blocks (Mou)

![ping-pong](https://cloud.githubusercontent.com/assets/2796074/4940625/24e60ad6-65dc-11e4-8f58-0931d3fb7359.gif)