Convert 1 dimensional iterables returned to .apply() columns to pd.Series
As per http://stackoverflow.com/questions/30621887/return-a-column-vector-in-pandas-apply-with-different-length/30622209#30622209

It would be nice if an apply function automatically converted lists to pd.Series when they are returned to .apply(), in other words,

``` python
import numpy as np
import pandas as pd

df = pd.DataFrame({'a':[0,10], 'b':[0,30]})

df.apply(lambda x: pd.Series(np.percentile(x, q=[10,20,30,40,50,75,100])), axis=0)
```

could become

``` python
df.apply(np.percentile, axis=0, q=[10,20,30,40,50,75,100])
```

(np.percentile currently returns a list, unless `axis=` is supplied, and then it returns a list of arrays o_O )
