Add CakeRequest::param()

This method gives a read accessor to the data in $request->params. It removes the need to use isset() and empty(). I felt like this would be a useful method when I had to write 

```php
if (!empty($this->request->param['admin'])) {
...
}
```

Which would just be:

```php
if ($this->request->param('admin')) {
...
}
```

After this change.  This method also completes the `data()` and `query()` cycle of methods.