Series.at and DataFrame.at crash with CategoricalIndex
#### Code Sample, a copy-pastable example if possible

```python
Python 3.6.3 (default, Oct  3 2017, 21:45:48) 
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> x = pd.Series([1, 2, 3], index=pd.CategoricalIndex(["A", "B", "C"]))
>>> x.loc["A"]
1
>>> x.at["A"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/indexing.py", line 1869, in __getitem__
    return self.obj._get_value(*key, takeable=self._takeable)
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/series.py", line 929, in _get_value
    return self.index.get_value(self._values, label)
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/indexes/category.py", line 423, in get_value
    return series.iloc[indexer]
AttributeError: 'numpy.ndarray' object has no attribute 'iloc'
>>> x = pd.DataFrame([[1, 2], [3, 4], [5, 6]], index=pd.CategoricalIndex(["A", "B", "C"]))
>>> x.loc["B", 1]
4
>>> x.at["B", 1]
Traceback (most recent call last):
  File "pandas/_libs/index.pyx", line 139, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 811, in pandas._libs.hashtable.Int64HashTable.get_item
TypeError: an integer is required

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/indexing.py", line 1869, in __getitem__
    return self.obj._get_value(*key, takeable=self._takeable)
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/frame.py", line 1985, in _get_value
    return engine.get_value(series._values, index)
  File "pandas/_libs/index.pyx", line 83, in pandas._libs.index.IndexEngine.get_value
  File "pandas/_libs/index.pyx", line 91, in pandas._libs.index.IndexEngine.get_value
  File "pandas/_libs/index.pyx", line 141, in pandas._libs.index.IndexEngine.get_loc
KeyError: 'B'
```

#### Problem description

With a `CategoricalIndex`, `Series.at` raises `AttributeError` and `DataFrame.at` raises `TypeError` and `KeyError`.

#### Expected Output

`x.at["A"]` should work the same as `x.loc["A"]`, and `x.at["B", 1]` should work the same as `x.loc["B", 1]`.

#### Output of ``pd.show_versions()``

<details>

commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-37-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.4.0
pip: 9.0.3
setuptools: 39.0.1
Cython: None
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: None
sphinx: 1.6.7
patsy: 0.4.1
dateutil: 2.7.2
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: 4.0.0
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.0
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
