Index astype('category') does not return a CategoricalIndex
#### Code Sample, a copy-pastable example if possible

```
In [1]: import pandas

In [2]: idx = pandas.Index(['a', 'b', 'c'])

In [3]: idx
Out[3]: Index(['a', 'b', 'c'], dtype='object')

In [4]: idx.astype('category')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-b8d97d97d03f> in <module>()
----> 1 idx.astype('category')

C:\...\pandas\indexes\base.py in astype(self, dtype, copy)
    889     @Appender(_index_shared_docs['astype'])
    890     def astype(self, dtype, copy=True):
--> 891         return Index(self.values.astype(dtype, copy=copy), name=self.name,
    892                      dtype=dtype)
    893

TypeError: data type "category" not understood
```

#### Problem description

The documentation for this method reads:

> Create an Index with values cast to dtypes. The class of a new Index is determined by dtype.

Since there is a CategoricalIndex type, it is reasonable for a user to expect that `.astype('category')` would return a CategoricalIndex object.

As a workaround for the issue, users can construct a CategoricalIndex directly:

```
In [7]: pandas.CategoricalIndex(idx)
Out[7]: CategoricalIndex(['a', 'b', 'c'], categories=['a', 'b', 'c'], ordered=False, dtype='category')
```

#### Expected Output

The method should return a CategoricalIndex equal to the following:
```
In [5]: pandas.CategoricalIndex(['a', 'b', 'c'])
Out[5]: CategoricalIndex(['a', 'b', 'c'], categories=['a', 'b', 'c'], ordered=False, dtype='category')
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.5.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.1
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.11.2
scipy: 0.18.1
statsmodels: 0.6.1
xarray: 0.8.2
IPython: 5.1.0
sphinx: 1.4.8
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.7
blosc: 1.5.0
bottleneck: 1.2.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.3
html5lib: 0.999
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.1.3
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: 2.43.0
pandas_datareader: None

</details>
