make api node style

[plrthink]

By making callback node style, the first argument of callback is error, users can use something like bluebird.promisify to convert the callback style to the promise style.

[darkrishabh]

This is a good change but it might be a breaking change for many. I am kind of involved with some other stuff these days will merge it soon. untill then you can fix/ make sure it doesn't break the current applications

[kylegoetz]

It's been two years and this hasn't been merged. For anyone coming to this who only knows how to promisify using Bluebird or another third-party promisify function, here's how you do it with the add function as an example:

function myAdd(item) {
    return new Promise(resolve => db.add(item, result => resolve(result)))
}

myAdd will now return a promise that resolves to what db inserted.