Ensure we can run on 5.2

Adds compatibility for 5.2. Simply use `HM_Backdrop_Task` instead of `HM\Backdrop\Task`.

Works perfectly on 5.3+ as well, since we actually have a) namespaces, and b) late static bindings. Requires a small workaround for subclassing Server on 5.2.
I get a fatal error as `namespace.php` doesn't exist...