Fixes asynchronous API

[brendanashworth]

In many of the functions in bcrypt.js, there was a lackluster
adherence to the concept of sync OR async APIs: in many of the
supposedly asynchronous functions, the callbacks would be called
synchronously, when they should always be called asynchronously.

See:

• http://blog.ometer.com/2011/07/24/callbacks-synchronous-and-asynchronous/
• http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony
• https://www.joyent.com/developers/node/design/errors

With this commit, this has been fixed by appropriately using
process.nextTick to asynchronously call the callback instead.

[defunctzombie]

meh, I don't think this matters that much. I am aware of those blog posts but don't really buy the argument that you have to do it that way. In my experience it has not led to confusion.

[brendanashworth]

Well, the API gives both synchronous and asynchronous versions of itself. It doesn't make much sense to have the sync API be completely sync and then async API to be part async and part sync... it should be one or the other.

Also, some applications rely on completing code after the asynchronous function is launched but before a callback is invoked, and invoking the callback synchronously would break this promise between the API, which claims itself to be asynchronous, and the actual implementation, which is synchronous.

Regardless, it is specified by Joyent to be the preferred standard for APIs. If you think that the errors should still be handled synchronously rather than asynchronously, we can always instead resort to throw new TypeError('...'); in the code, which is what many other libraries do (see: fs.js).

[defunctzombie]

Callback API simply means that the callback can happen at a later time and that you do not know exactly when. I personally do not feel that strongly about it nor do I think it has been an issue for uses of this library. If @ncb000gt feels differently we can merge this. I think it is not useful.

[ncb000gt]

Although I don't, nor do most people using this module afaict, find this to be an issue, I don't mind merging this in. It does make the async api purely async and that's probably a good thing overall.

[brendanashworth]

Thanks!