Create julia install script

Closes https://github.com/JuliaLang/julia/issues/7364 - mostly intended for use on Travis and similar CI services, so you can do something like `curl http://julialang.org/install-julia.sh | sh -s $JULIAVERSION` and have it work for both Linux and Mac if you've enabled multi-OS support.
slight refactoring

account for renamed generic binary link
there are now 32-bit generic linux binaries
have stable linux binaries now too
Merge pull request #151 from JuliaLang/tk/install-julia-script

RFC: Create generic julia install script