Add a prepublish script

I couldn't find a documented release process so apologies if this has already been discussed.

I would like bluebird to define a "prepublish" script that runs the default build so that `npm install petkaantonov/bluebird#{commit-ish}` will work correctly. Despite being somewhat unintuitive, this is the recommended/only way to accomplish that goal.
Merge pull request #290 from grncdr/patch-1

Add a prepublish script