move stream into it's own file

[dominictarr]

Since you generally get a dataurl in buffered in full, streams are generally overkill.
Also, streams make this 100k larger when browserified.
(since stream3 now have a lot of code, which wasn't true in 2013 when this module was last updated, though it's still 50k, because of Buffer)

So I suggest moving streams into it's own submodule. require('dataurl').stream is now at require('dataurl/stream').

Also, I added a stringify as an alias to format. I think this is easier to remember because it's just like the familiar JSON.stringify.

This is a breaking change, so should be published as 1.0.0