Add the ability the define inherited hooks.

Creating inheritance hooks that play well with others (e.g. don't
clobber inheritance that are already defined) requires injecting
inheritance hooks into the Class class (via extend). This works pretty
well, but requires a lot of setup when typically, only a few lines need
to be executed (i.e. eval'd) on the subclass.

To simplify this add, an `on_inherited` method that captures code and
executes when the class is inherited.
Merge pull request #2 from liveh2o/master

Add the ability the define inherited hooks.