Basic/naive spy implementation

Had a play with this tonight, basic use:

``` php
<?php

$spy = m::spy();
$spy->myMethod();
$spy->shouldHaveReceived("myMethod");
```

It wont handle some weird cases, because it tries to verify on the fly, i.e., you don't have to manually call a `verify` method. Having such a requirement would make it possible to do 

``` php
<?php

$spy->shouldNotHaveReceived("find")->with(123)->verify();
```

However, I'm not really sure I'd ever really write code that would require that, and even if I did, I could always fall back to mocks with expectations.

Would appreciate thoughts, I'm keen to get some kind of spy implementation in soon so people can give it a good look before we settle on an API for 1.0.