Setup development environment with Vagrant

Vagrantfile.example is provided in contrib directory. To setup the development environment, copy it to project root directory and name it Vagrant, then run command:

vagrant up --provider virtualbox

After vagrant succeeds to run the virtual machine, you will get a complete environment to develop Nitrate,

a Python virtual environment created at $HOME/nitrate-env/ with all necessary dependencies installed.
a superuser is created by default with username admin and password admin. It is free for you to modify user’s properties from Django admin WebUI.
source code is mounted at /code.
database is created in MariaDB and name is nitrate. It’s empty. Before hacking and running development server, remember to synchronize database from models from /code.
```
./manage.py migrate
```
port forwarding. 8000 is mapped to 8087 in host.
Run development server from /code
```
./manage.py runserver 0.0.0.0:8000
```

visit http://127.0.0.1:8087 with your favorite web browser.

Happy hacking.