Multiple world instances in javascript

[EJTH]

I don't know wether this is a bug or a feature, but I cannot seem to make anything work unless I have my b2World in a global variable named 'world' when using the javascript compiled version of liquidfun.

[zilluss]

I already mentioned this in #46 and I'll make a PR so you can access the world from a b2Body (which is an important part of the default API anyway).