BUG: fix Categorical comparison to work with datetime
`pd.Timestamp` is one of types that don't pass `np.isscalar` test previously
used in _cat_compare_op, `pd.lib.isscalar` should be used instead.

``` python
In [1]: cat = pd.Categorical(pd.date_range('2014', periods=5))

In [2]: cat > cat[0]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-77883e4783fe> in <module>()
----> 1 cat > cat[0]

/home/dshpektorov/sources/pandas/pandas/core/categorical.py in f(self, other)
     52             msg = "Cannot compare a Categorical for op {op} with type {typ}. If you want to \n" \
     53                   "compare values, use 'np.asarray(cat) <op> other'."
---> 54             raise TypeError(msg.format(op=op,typ=type(other)))
     55 
     56     f.__name__ = op

TypeError: Cannot compare a Categorical for op __gt__ with type <class 'pandas.tslib.Timestamp'>. If you want to 
compare values, use 'np.asarray(cat) <op> other'.

```

can u add a simple doc note ref this pr
 Ah, indeed, I've forgotten about that.
 thanks!
