node: add slim variant

```
$ docker images node
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
node                0.10                ae023fecd24f        3 days ago          714.9 MB
$ docker images proppy/node
REPOSITORY          TAG                 IMAGE ID            CREATED              VIRTUAL SIZE
proppy/node         0.10                b50f869976fa        About a minute ago   294.9 MB
```

Main changes are:
- use wheezy instead of buildpack-deps
- add --no-install-recommends with more explicit build deps (pkg-config, python)
Ok, fair enough - good starting point at least. :+1: