Don't manually inherit .init, use prototype chain #ImAnIdiot

Prior to this commit, would copy `.init` from superclass to each
subclass, for example if after creating the subclass you did `.open()`
and changed a bunch of methods on the superclass including `.init`,
all the methods except `.init` would change on the subclasses.

https://github.com/laughinghan/pjs/commit/0803fa5e3e8ac79a575375ab0efb0973e17acfc9#commitcomment-4445265