cleaned up HTTPResponse.php

Oh my god, everything I've known up until now has been wrong.

http://www.phpdoc.org/docs/latest/guides/types.html#keywords

You're right and I tested it. self locks to the class the method is defined in, not respecting late static binding.

```php
class bob extends SS_HTTPResponse {
	public function doBob() {}
}
$var = new bob();
$var
	->setStatusCode(404)
	->doBob(); // intellisense detects with @return static on setStatusCode(), but not on self
```