pre-build libraries for Julia packages and create default git config

Build and install native dependencies of bundled Julia packages in base image. Reduces build time and space requirement for Julia packages (Dockerfile v13).

Also generating a default git config for users on first login. 

Not in any currently released version of Ubuntu, no. Cbc.jl had been using CoinMP which is an extra C API layer on top of Cbc, but had its own set of bugs, problems, and limitations. @mlubin wrote a brand new C API specific to Cbc which is only incorporated in the just-released version 2.8.12, and switched the package over to use it. Miles is also the Debian maintainer for Cbc and he uploaded the newest version very shortly after release, I think it will make the cut before the next version of Debian freezes, but don't think it'll be in Ubuntu 14.10. Maybe 15.04.