Added autoloader for those poor non-composer peeps

# Usage

If you don't have composer, you can still autoload the SDK like so:

```php
require __DIR__ . '/path/to/facebook-php-sdk-v4/autoload.php';
```

It should detect the `src/` folder automagically for you. If not you can define a constant with the path to the source files before the include.

```php
define('FACEBOOK_SDK_V4_SRC_DIR', '/path/to/facebook-php-sdk-v4/src/Facebook/');
require __DIR__ . '/path/to/facebook-php-sdk-v4/autoload.php';
```

Also threw in a little PHP version check in there since that seems to be a recurring issue. People are getting syntax errors on < 5.4 and don't get a clear error message saying they need 5.4 or greater.

I didn't add this info the the README since the composer installation stuff wasn't there either, so maybe we could add it to the official documentation?