Joining DataFrames (or Series) and Series.
```
pd.version.version
'0.12.0-1149-g141e93a'
```

Maybe already fixed, but I'm afraid to upgrade in the middle of working on a project given past experiences.

As I recall there were several ways proposed to join a series to a DataFrame with join being the preferred way. 

```
df = pd.DataFrame(np.random.random((30,2)), columns=["A","B"])
series = pd.Series(np.random.random(30), name="C")
```

I know I can do 

```
df["C"] = series
```

or 

```
df = df.join(series)
```

But it'd be nice if the following also worked given that I'd like to write general code.

```
pd.concat((df, series), axis=1) # ok error message at least
df.merge(series) # just call join under the hood? bad error message
```

This is "unexpected" that it somehow tries to append the rows. Looks like a bug to me. Should it take an axis keyword?

```
df.append(series)
```
