Format Dockerfile following best-practices

In particular, we want to run `apt-get update` and `apt-get install` on the same line to avoid caching issues.  
Also reduces the number of AUFS layers as recommended, https://docs.docker.com/articles/dockerfile_best-practices/

A few additional things might be worth considering(?):
- Why ubuntu:14.04 by default?  A Debian base, as mentioned in the article, would be considerably smaller.
- perhaps consider COPY in place of ADD

(apologies for such pedantic edits!)
Merge pull request #37 from cboettig/master

Format Dockerfile following best-practices