Fix for freezing function objects in strict mode

The current implementation of `deep-freeze` blows up in strict mode when deep-freezing a function or an object structure having a function property.
For example:

```js
"use strict";
var deepFreeze = require('deep-freeze');
var obj = {
  callback: function (){},
};
deepFreeze(obj);
```
throws:
```
TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them
```

This PR avoids the error by skipping those properties. It also actually slightly improves performance in some cases (due to not freezing the skipped properties).