Association are taking off, another step to Invoices

#15 #16 #17 #18 #19
Styling standard:

Objects always have the colon on the end of the property name, and they are aligned like so:

```javascript
var obj = {
  foo      : 'value',
  bar      : 'value',
  bat      : 'value',
  bacon    : 'value',
  something: 'value',
  baz      : 'value'
};
```