deleting last column of ctable raises TypeError

[suntzu86]

If we are deleting the last column, then self.dtype == numpy.dtype([]) and so creation of arr1 fails:
https://github.com/Blosc/bcolz/blob/master/bcolz/ctable.py#L590

    589         # Update _arr1 for the new dtype
--> 590         self._arr1 = np.empty(shape=(1,), dtype=self.dtype)
    591
    592         if not keep:

TypeError: Empty data-type

This means purge and flush are never called on the deleted column.

Example:

test_ctable = bcolz.ctable(numpy.zeros(2, dtype='f8, i4'))
test_ctable.delcol('f0')
test_ctable.delcol('f1')  # fails with the above error

Probably not too critical since I imagine it's uncommon to delete everything like this.

Note: deleting the last col further puts the object in a broken state. This is the output from trying to print test_ctable after I ran the above 2 delcol statements.

In [671]: test_ctable
Out[671]: ---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
virtualenv_run/lib/python2.7/site-packages/IPython/core/formatters.pyc in __call__(self, obj)
    697                 type_pprinters=self.type_printers,
    698                 deferred_pprinters=self.deferred_printers)
--> 699             printer.pretty(obj)
    700             printer.flush()
    701             return stream.getvalue()

virtualenv_run/lib/python2.7/site-packages/IPython/lib/pretty.pyc in pretty(self, obj)
    381                             if callable(meth):
    382                                 return meth(obj, self, cycle)
--> 383             return _default_pprint(obj, self, cycle)
    384         finally:
    385             self.end_group()

virtualenv_run/lib/python2.7/site-packages/IPython/lib/pretty.pyc in _default_pprint(obj, p, cycle)
    501     if _safe_getattr(klass, '__repr__', None) not in _baseclass_reprs:
    502         # A user-provided repr. Find newlines and replace them with p.break_()
--> 503         _repr_pprint(obj, p, cycle)
    504         return
    505     p.begin_group(1, '<')

virtualenv_run/lib/python2.7/site-packages/IPython/lib/pretty.pyc in _repr_pprint(obj, p, cycle)
    692     """A pprint that just redirects to the normal repr function."""
    693     # Find newlines and replace them with p.break_()
--> 694     output = repr(obj)
    695     for idx,output_line in enumerate(output.splitlines()):
    696         if idx:

virtualenv_run/lib/python2.7/site-packages/bcolz/ctable.pyc in __repr__(self)
   1394
   1395     def __repr__(self):
-> 1396         nbytes, cbytes, cratio = self._get_stats()
   1397         snbytes = utils.human_readable_size(nbytes)
   1398         scbytes = utils.human_readable_size(cbytes)

virtualenv_run/lib/python2.7/site-packages/bcolz/ctable.pyc in _get_stats(self)
   1387             nbytes += column.nbytes
   1388             cbytes += column.cbytes
-> 1389         cratio = nbytes / float(cbytes)
   1390         return (nbytes, cbytes, cratio)
   1391

ZeroDivisionError: float division by zero