fix for use in strict mode

When observe-js is included in a strict mode wrapper function, `this` is undefined thus the `global` variable in the observe-js module wrapper function is `undefined`. An exception is thrown on the `global.Number` line at the top (cannot access `Number` on `undefined`).

```javascript
(function () {
  'use strict';
  
  // observe-js code here

  // application code here
}());
```