Split up into multiple images

Splitting up the images gives you the ability to run a hub and node in
separate containers. So you can run a hub with:

```
docker run -d --name selenium-hub elgalu/selenium-hub:2.44.0
```

Then you can run as many nodes as you want with:

```
docker run -d --link selenium-hub elgalu/selenium-node-chrome:2.44.0
docker run -d --link selenium-hub elgalu/selenium-node-firefox:2.44.0
```

Or if you want to stick with the previous model with a combined hub/node
container you can run:

```
docker run -d elgalu/selenium-full:2.44.0
```