cache database connection in module space

I like this future from mongoose as you do not need to handle dependency injection.

```
var monk = require('monk');

// connection still the same
monk('localhost/test');

// however this will return collection
monk.get('users');
```

What do you think?

The issue contains only a proposed module-level connection and collection API using monk and MongoDB; no files, tests, or entry points are named. Start by locating the connection and collection access APIs, then determine whether this design fits the project before defining tests and acceptance criteria.

Written by the indexing model from the issue text.