switched nedb to nedb-promises (#37)

* breaking change: switched to nedb-promises
* Linting
* some dev dependencies
* more tests
* updated package version
* bug fixing
* changed isNaN to isString
* (fix) args[0]

functions/isEmpty.js

@@ -0,0 +1,6 @@
+function isEmpty(document = {}) {
+    if (!document) return true;
+    if (document.length === 0) return true;
+    return Object.keys(document).length === 0 ? true : false;
+}
+module.exports = { isEmpty };