RESTful one-to-one associations

This pull request adds RESTful one-to-one associations, now allowing both definitions like:

```js
// Both associations should only work if there is a /users service registered already
app.use('/users', userService)
  .use('/posts', postService)
  .use('/accounts', accountService);

// Pass service name in an array
// Calls postsService.findAll({ userId: <userId> })
app.associate('/users/:userId/posts', ['posts']);

// Calls userService.get(<id>) then calls
// accountService.get(user.account)
app.associate('/users/:id/account', 'accounts');
```