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/isString.js

@@ -0,0 +1,4 @@
+function isString(str) {
+    return typeof str === 'string' || str instanceof String ? true : false;
+}
+module.exports = { isString };