base class

hi,

as discussed, this is the base class implementation. at the moment this primarily only exposes an empty function with a single "extend" function. this allows you to use an object API, with prototypal inheritance.

i copied the extend function from Backbone, just for its maturity. to be 100% honest i'm not sure of the implications here, or if we should simplify it to something like the existing extend function.

there is a breaking change introduced from v1: the original `extend` function has been removed from Audiolet. although the previous extend function will continue to work fine on existing Audiolet classes, if that is what someone would prefer. but, they will have to BYO extend.