Series.str.cat(',') without "sep" keyword arg raises unintuitive `TypeError: len() of unsized object`
I think this easy-to-make mistake should raise a more descriptive error... maybe something like 

``` python
ValueError: Did you mean to supply a 'sep' keyword?
```

I might be missing the main usage of `str.cat()`, but all the uses I've found involve concatenating series together with a sep character.

``` python
In [1]: import pandas as pd

In [2]: s = pd.Series(['foo', 'bar', 'baz'])

In [3]: s.str.cat('|')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-8bc5f2a1347b> in <module>()
----> 1 s.str.cat('|')

/home/charles.hack/anaconda/envs/seahydra/lib/python2.7/site-packages/pandas/core/strings.pyc in cat(self, others, sep, na_rep)
   1137     @copy(str_cat)
   1138     def cat(self, others=None, sep=None, na_rep=None):
-> 1139         result = str_cat(self.series, others=others, sep=sep, na_rep=na_rep)
   1140         return self._wrap_result(result)
   1141

/home/charles.hack/anaconda/envs/seahydra/lib/python2.7/site-packages/pandas/core/strings.pyc in str_cat(arr, others, sep, na_rep)
     71         arrays = _get_array_list(arr, others)
     72
---> 73         n = _length_check(arrays)
     74         masks = np.array([isnull(x) for x in arrays])
     75         cats = None

/home/charles.hack/anaconda/envs/seahydra/lib/python2.7/site-packages/pandas/core/strings.pyc in _length_check(others)
    111         if n is None:
    112             n = len(x)
--> 113         elif len(x) != n:
    114             raise ValueError('All arrays must be same length')
    115

TypeError: len() of unsized object

In [4]: s.str.cat(sep='|')
Out[4]: 'foo|bar|baz'
```
