don't use more than 8 processes to run tests, to save memory on build workers
make searching for an editor more flexible

- use "start" or "open" as the default on Windows and Darwin
- use /etc/alternatives/editor by default on unix if it exists (fixes #2966)
- check EDITOR env var if JULIA_EDITOR is not set
- finally, if an unknown editor is set, just run it
Merge branch 'master' of github.com:JuliaLang/julia