Avoid dependency on global document.

[Matt-Esch]

Carried over task from vdom:

I would like to use this library in the context where there is no single global or document (It is a browser code that has access to every document loaded, but there is not a single one that we could pick) there for it isn't possible to provide something like require("global/document").

As far as I see createElement without document, is just for a convenience of the API, if it's really crucial maybe some other package could wrap this createElement to do that.

Matt-Esch/vdom#15