Display a summary if there are more than 15 columns. Fixes #373.

Here's a start. Now the output when there are more than 15 columns is something like:

```
DataFrame with 10000 rows, 52 columns
Columns:

Unique Key                          10000 non-null values    
Created Date                        10000 non-null values    
Closed Date                          4247 non-null values    
Agency                              10000 non-null values    
Agency Name                         10000 non-null values    
Complaint Type                      10000 non-null values    
Descriptor                          10000 non-null values    
Location Type                        6354 non-null values    
Incident Zip                         8633 non-null values      
```

15 is chosen kind a bit arbitrarily -- it would be better to figure out the terminal width and make a smart decision.

I've also added a new function `column_summary` which isn't the best since there's already `summarize` and `dump`