Refactor workspace:

Use Open3 instead of popen4
Remove rvm init (it should inherit rvm/rbenv from calling environment)
Refactor way workspaces are prepared (no longer uses stash business)
Merge fork_and_execute functionality into execute
Use Open3 when fetching hostname. Open3 will call Process.detach, so the curl process cannot become a zombie.
Use new process group when executing commands so that they can be killed as one.
Bump to version 0.0.2
don't clog filesystem by default