Use level-manifest instead of hard-coding

[edef1c]

Hardcoding the set of methods makes it impossible to apply this to LevelUp extensions too.
https://github.com/nathan7/level-promise uses level-manifest instead, so it works with any LevelUp extension that adheres to the level-manifest spec.

[ForbesLindesay]

I've added you as an owner npm. If you want to release new versions, feel free. I'm not actively using this so I've been a bad steward of it so far.

[abacaj]

You don't need this anymore.

Just use bluebird promisify save file as db.js:

Promise = require('bluebird');
const levelup = require('level');

function createDb(dbPath) {
	let db = levelup(dbPath);
	return Promise.promisifyAll(db);
}

module.exports = createDb('./mydb');

and put values like so:

const db = require('./db');
db.putAsync('key','value').then(() => {});

or read values like so

const db = require('./db');
db.getAsync('key')
	.then((e) => {
		console.log(e);
	})
	.catch((err) => {
		console.log(err);
	})