Added CentOS support

Also added a nice little badge to the readme.
As mentioned on Slack, we distribute Python 2.6 from rpm.nodesource.com when you install *nodejs* from there, it uses EPEL to pull in a dependency of Python.

The problem you're probably facing is that you're invokine `node-gyp` without telling it what Python to use, we have hacked `npm` in the EL5 distro to go directly to `python26`, what you may need to do in the Dockerfile is:

```
RUN PYTHON=python26 node-gyp configure || echo ""
```

Either that, or just `npm install buffertools`, then clean up afterwards to achieve the same thing and `npm` will pull in the right version of Python.