lib.isnullobj fails on list of strings input
Was playing around with the new string functions and noticed this. Don't really know if it's a bug or expected

```
np.version.full_version                                                     
#'1.6.1'
df = pandas.DataFrame(['jack and jill','jesse and frank'])
split_df = df[0].str.split(r'\s+and\s+')
split_df.to_csv('test')
#<snip>
#    75         shape = values.shape
#     76         result = np.empty(shape, dtype=bool)
#---> 77         vec = lib.isnullobj(values.ravel())
#     78         result[:] = vec.reshape(shape)
#     79 
#
#/usr/local/lib/python2.7/dist-packages/pandas-0.8.2.dev_3942eb0-py2.7-linux-x86_64.egg/pandas/lib.so in pandas.lib.isnullobj (pandas/src/tseries.c:5355)()            
#
#ValueError: Does not understand character buffer dtype format string ('s')

from pandas import lib
lib.isnullobj(np.array(split_df.ix[0]))                                      
#<snip>
#
#ValueError: Does not understand character buffer dtype format string ('s')
```
