[feature]zekai&zhihao: job test included
[feature]zekai&zhihao: add job model for both front end and backend
[feature]zekai&zhihao: Integrate job models to Jobs App

1. move database instance to a single module for other module to ref
2. add jobs-cruder implementation for Jobs model to support mongokin
3. Change express middleware req.db to Jobs model to do CRUDs