Feature request: pandas.DataFrame.setdefault()
We often find ourselves checking for column existence before assigning a value to it, e.g.:

``` python
df = pd.DataFrame()
col = 'ColumnA'
val = 'Value1'
if col not in df.columns:
    df[col] = val
```

And we were thinking it would be nice if there was a way to tell pandas: add this column with that value(s) but only if it does not exist already - somewhat similar to dict.setdefault() so we could say, for instance:

``` python
df.setdefault(col, val)
```

Doesn't necessarily have to be that syntax obviously - just a suggestion. What do you guys think?
