Should pandas preserve NaNs in str methods?
Since the output of the string methods is primarily boolean, do we want to preserve NaNs? Use case. I load in some data, empty strings (i.e., missing data in a column that's string data) gets automatically converted to NaNs. I have to do fillna everywhere first, or I run into situations like this. 

```
dta = pd.DataFrame([["blah"],[np.nan],["foobar"]], columns=['vari'])
dta.vari.str.startswith('f')
dta.ix[dta.vari.str.startswith('f')]
```

Since presumably the primary use case of a boolean series is indexing, do we want to just return False in place of NaNs? Just a thought.
