[FEA] Implement pandas.Categorical

**Is your feature request related to a problem? Please describe.**
While porting some existing code to cuDF, I have notices `pandas.Categorical` method is not implemented yet.

**Describe the solution you'd like**
To have it implemented.

**Describe alternatives you've considered**
I have used the following workaround it as follows (column 'D'):

```
df = cudf.DataFrame({'A': 1.,
'B': 2,
'C': np.array([3] * 4, dtype='int32'),
'D': cudf.Series(["cat", "dog", "lion", "fish"], dtype='category'),
'E': 'foo'})

```

**Additional context**
Using RAPIDS 0.13