WIP: add Path string type

[nolta]

This might be a little too cute, but i've been playing around w/ adding a Path string type:

julia> p = path("a", "b")
"a/b"

julia> typeof(p)
Path

julia> isa(p, String)
true

julia> q = p / "c"
"a/b/c"

julia> split(q)
3-element String Array:
 "a"
 "b"
 "c"

Inspired by https://github.com/jaraco/path.py

[StefanKarpinski]

That is cute. I'll have to play with it once I'm back from Puerto Rico on Sunday. Let's see where it goes and how it feels. Might be rather nice for writing portable code more easily.

[staticfloat]

Glad to see this is being made! Back in #1396 I suggested using abs_path in the String constructor of a class like this. In this case, it would be put in base/file.jl:42, such that if you were to do something like:

p = Path("/tmp/adir/") / ".." / "bdir/"

p would end up with /tmp/bdir/, not /tmp/adir//../bdir/. The nice thing about the way you've put this all together, is that any way a user combines String and Path objects will need to pass through that method, so the data inside the Path object can be "trusted" in a sense, that we know it has already been passed through abs_path and therefore is safe to pass to naive methods that don't want to worry about things like // and /../.