Updated specs to be compatible with the latest phpspec version.

Appart from changing the spec file names and updating method names, I also needed to make few changes in the `src` folder:

* `src/Prophecy/Argument/Token/ObjectStateToken.php` - changed the `is_callable()` call to `method_exists()` since `is_callable()` [returns true](http://www.php.net/manual/pl/function.is-callable.php#49879) if `__call()` is prevesent
* `src/Prophecy/Doubler/Generator/ClassMirror.php` added a check if an argument allows `null` values (for arguments with default `null` value).
* `src/Prophecy/Exception/Exception.php` - Added the `getMessage()` method to the `Exception` interface.