Fixed compatibility with the actual implementation of hump.class

Hi Headchant,

Since [hump.class](https://github.com/vrld/hump/blob/master/class.lua) had some breaking changes, here is  a quick fix for compatibility with the latest version. I just had to change the way constructors were passed to the class creation function (tables with an explicit `init` field, instead of plain functions).

Regards,
Roland.