[4.1] Added helpers to super service. Added setter for default access token.

Just a bit more cleanup before really hitting the docs.

Added getters for the rest of the helpers:

```php
$helper = $fb->getRedirectLoginHelper(); // Already existed
$helper = $fb->getJavaScriptHelper();
$helper = $fb->getCanvasHelper();
$helper = $fb->getPageTabHelper();
```

Added a setter for the default access token. This is handy when you want to use a default access token but don't have it when you instantiate `Facebook\Facebook`.

```php
$fb = new Facebook\Facebook([ /* */ ]);
// . . . Get access token here . . .
$fb->setDefaultAccessToken($accessToken);
```

Also uncovered a bug that wouldn't set the default access token if you passed in a `Facebook\Entities\AccessToken`. Yikes! Wrote some tests to cover that case. :)