stat: don't fail on stat("foo/bar") when "foo" is not a directory.

Also, allow calling stat-related functions with varargs by doing a
joinpath on the pieces and then stat:ing that path.
produce(x,y): with multiple args, produce the tuple of arguments.
wip: full re-write of Pkg functionality.
Merge branch 'master' of github.com:JuliaLang/julia