Added getValues() method to allow read-only access to the private values array

When extending the Pimple class to provide additional functionality I was previously accessing `$this->values` directly when it was protected. As I was only reading the data, adding a getter (as I've done in this PR) is perfectly fine but I think it's important to expose this array in order to allow extensions.

Note: I realize it would be possible to access the information by using the public `keys()` method and then iterating for each key and requesting it's value, however this would have a significant performance impact.