Option for crosstab/pivot_table to include empty columns
I don't think you can do this easily atm. _crosstab is just a [convenience for pivot_table]((https://github.com/pydata/pandas/blob/master/pandas/tools/pivot.py#L218)..._

http://stackoverflow.com/questions/17003034/missing-data-in-pandas-crosstab

```
a = np.array(['foo', 'foo', 'foo', 'bar', 'bar', 'foo', 'foo'], dtype=object)
b = np.array(['one', 'one', 'two', 'one', 'two', 'two', 'two'], dtype=object)
c = np.array(['dull', 'dull', 'dull', 'dull', 'dull', 'shiny', 'shiny'], dtype=object)

pd.crosstab(a, [b, c], rownames=['a'], colnames=['b', 'c'])
b     one   two       
c    dull  dull  shiny
a                     
bar     1     1      0
foo     2     1      2

# but wants
b     one        two       
c    dull  shiny dull  shiny
a                     
bar     1     0    1      0
foo     2     0    1      2
```

Maybe we could have an option in pivot_table (and crosstab) to include empty columns.
