[5.0] Add session helper functionality

...to bring it more in line with the `config` helper:

```php
session('key', 'default') // Session::get('key', 'default')

session(['key' => 'value']) // Session::set('key', 'value')

// Calling session() returns the store instance. Do with it whatever you want...

session()->forget('key') // Session::forget('key')
```
I'm just following everything else in here:

https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L42
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L76
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L137
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L158
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L161
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L162
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L267
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L268
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/helpers.php#L391

I can go on and on... It is like this throughout the framework.