Introducing property access in Pimple

This PR introduces the concepts at the foundation of [YAC](https://github.com/asm89/yac) into Pimple.

This basically allows to use pimple like following:

```php
$pimple = new Pimple();
$pimple['someService'] = ...;

$pimple->someService->doStuff();
$pimple['someService']->doStuff(); //equivalent
```