Adds a __str__ and __unicode__ method to the Column model

Here we can discuss also the format for that ```__str__``` representation of the object.

I took the "declutering exception handling" branch as a base branch because otherwise it would blow.

The format I chose for the string representation of the ```List``` model was

```account-id: column_slug```

An example could be

```iferminm-twitter: timeline```