[WIP] Add Julia package (Debug)

**DO NOT MERGE**

This is a possibly hackish PR, where we iteratively test ideas and nail things down. 
A nice, polished PR is #197, where we push good ideas from here, that are known to work.

TODO:

* [ ] make it work with libraries from hashdist
* [ ] disable downloading of `dsfmt-2.2.tar.gz`

The best is to look at how Homebrew does it: https://github.com/staticfloat/homebrew-julia/blob/master/julia.rb

One can also look at how Debian does it: https://packages.debian.org/sid/julia, in particular, the file [julia_0.2.1+dfsg-2.debian.tar.xz](http://ftp.de.debian.org/debian/pool/main/j/julia/julia_0.2.1+dfsg-2.debian.tar.xz) shows how to patch Julia sources to do what we want.