Initial working version

* Ruby 2.1.5
* Node.js v0.10.33
* Python 3.2.3

It's a big image :disappointed: at 430.6 MB, but still smaller than the Ubuntu-based quay.io.aptible/ruby image (473 MB). Here's the relevant part of the image history; @aaw, thoughts on compressing it further? I could maybe install build-essential as part of the Ruby install and then remove it (and all its dependent packages as part of the same `RUN` instruction as installing Ruby)? Perhaps we could have two tags/branches: one for build-essential, one without?

```
$ docker history quay.io/aptible/webapp-essential
IMAGE               CREATED             CREATED BY                                      SIZE
6768b93f2d23        17 minutes ago      /bin/sh -c bats /tmp/test                       0 B
ba1538e3ee1a        17 minutes ago      /bin/sh -c #(nop) ADD dir:2e1d54bf59c08460261   352 B
50066358fba2        19 minutes ago      /bin/sh -c ln -s python3 /usr/bin/python        7 B
7281ed681734        19 minutes ago      /bin/sh -c apt-install python3-minimal          27.62 MB
03d5e3cea7f7        19 minutes ago      /bin/sh -c cd /usr &&     wget http://nodejs.   17.44 MB
db44c1f5abe2        19 minutes ago      /bin/sh -c gem install -N bundler               4.033 MB
98c9f7ab87a5        19 minutes ago      /bin/sh -c cd /tmp &&     wget -q http://cach   109.8 MB
ea8d1bc7134b        23 minutes ago      /bin/sh -c apt-install zlib1g-dev libssl-dev    10.6 MB
de3bf6ea9d67        24 minutes ago      /bin/sh -c apt-install build-essential git      165.2 MB
```