naive shell using ffi

I would like to use pixie to do some scripting.

Here is a crack at trying to implement a _shell_ function. It returns the exit code.

```
-> ./pixie-vm
Pixie 0.1 - Interactive REPL
(darwin_x86_64, clang)
:exit-repl or Ctrl-D to quit
----------------------------
user => (shell "ls")
COPYING			README.md		foo			pixie-vm		target.pyc
COPYING.LESSER		benchmarks		make-no-jit		run-interpreted		tests
Dockerfile		checkout-externals	make-with-jit		run-tests.pxi
Makefile		examples		pixie			target.py
0
user => 
```
```
user => (shell "mkdir 'foobar'")
0
```

I am definitely don't know my way around python, ffi, or libuv very well, so if there is a better way to implement this, please let me know.