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
```
Makefile cleanup

+ Renamed USER to NAME and reduced duplication of `/selenium` name
declaration
+ Separated tmp files for various images
+ Added clean task
+ Added tag_latest task
+ Added release task
Updated README, allowed configurable VERSION

+ Updated the README for the image split up changes
+ Allow version to be set by command line when invoking make