Updated README markdown so get color and syntax highlight 

@sbenthall : Great project. Would love to contribute. 

This is a simple pull request for a little nicer README, with syntax highlighting. I personally find them useful in code blocks not just for the color but comments get rendered as comments. 

For eg:

#### before

    $git clone https://github.com/sbenthall/bigbang.git
    # optionally create a new virtualenv here
    $pip install -r requirements.txt
    $python setup.py develop

#### after

```bash
$git clone https://github.com/sbenthall/bigbang.git
# optionally create a new virtualenv here
$pip install -r requirements.txt
$python setup.py develop
```