Export spawn as a property as well as a function.

This makes both of the following usages correct:

```javascript
var spawn = require('cross-spawn');
spawn(/**/);
```

```javascript
var proc = require('cross-spawn');
proc.spawn(/**/);
```

The motivation behind this is to make unit testing with libraries like [`sinon`](http://sinonjs.org/) easier. By making `spawn` a method of the module's resolved value we can easily stub it across the entire application like so:

```
var proc = require('cross-spawn');
sinon.stub(proc, 'spawn');
```

That way no changes to existing code is required in order to use a stubbed / spied on cross-platform spawn function.