Loss of series name attributes in pd.concat 
`pd.concat` nicely preserves name attributes when given a list of series. However, this only works if all series have the name attribute set, otherwise all names are lost:

```
In [1]: import pandas as pd
In [2]: pd.__version__
Out[2]: '0.16.2'

In [3]: foo = pd.Series([1,2], name='foo')
In [4]: bar = pd.Series([1,2])

In [6]: pd.concat([foo, bar], 1)
Out[6]: 
   0  1
0  1  1
1  2  2
```

Not sure this qualifies as a bug.
