Move usage example to the top of the file

I also slightly updated the example to clarify.
Make within() global in both browser and nodejs

Using var at the top level only creates a global variable in the browser,
not in node (the variable is scoped  in the file module).

Except in strict mode (when "use strict"; is found in the outer scope
of the function), "this" refers to the global object in a function called
directly or applied to a null object.

Changing
var within = privately(function(){
  ...
  return within;
});

to

privately(function(){
  ...
  this.within = within;
});

exports within to the global scope in both browser environment and
nodejs environment.
Extract unit tests to a separate file

The goal is to reuse the same tests in nodejs as in the browser.
Add unit test: "within is expected to be a global function"
Add script to run tests in nodejs

The script declares the global functions expected by within.test.js:
* log()
* test()
* assert()