Major reorganization. I was finding myself pausing and thinking about where things should go, so I've made it more obvious.
Move `settings.py` to the `piss` folder

As PISS becomes more developed, more things hinge on the specific structure of
the `posts` endpoint. As a result, I figured it would be a good idea to have
the `settings.py` file live inside the `piss` module.

However, it's still necessary to provide a means for changing MongoDb settings
and other things, so now specifying `EVE_SETTINGS` in `piss.cfg` will allow
users to provide an alternate settings file.

To create a settings file that adopts most of the defaults but overrides only
a few settings, the following is recommended:

```python
from piss.settings import *

MONGO_HOST = 'localhost'
MONGO_PORT = 27017
MONGO_USERNAME = 'user'
MONGO_PASSWORD = 'user'
MONGO_DBNAME = 'piss'
```