Allowing dot-notation arrays to be used in session data.

Hi,

Let's say you have the following form:

```php
<form>
    <input type="text" name="user[first_name]" value="{{ Input::old('user.first_name') }}">
</form>
```

This will not work under the current session implementation as the keys are limited to 1 deep, i.e. 'user' or 'user_first_name', but not 'user.first_name'. I have implemented `array_set`, `array_get` and `array_forget` (`illuminate/support` is already a dependency of `illuminate/session` so no issues there) to allow multi-dimensional arrays to be set.

There are several use-cases where you may wish to set / access multi-dimensional session data, HTML forms are but one of them.

 \- Ben