Multisort and default sort overrides

Two wins here:

- Allow for multiple sort rules
    - There is a fallback for using the old style `sortColumn` and `sortDirection`
- Override default sort behavior on first sort click
    - Teeble defaults to desc since most of our data lends itself to it, but now we can have asc columns by default
    - `sortDirections` defaults to an empty object, so you can just override the columns you care about

```coffeescript
class CoolCollection extends Teeble.ClientCollection
    sorting: [
        {column: 'startedAt', direction: 'desc'}
        {column: 'display_name', direction: 'asc'}
    ]
    sortDirections:
        display_name: 'asc'
```

@marcneuwirth @jessbrandi