Categorical.searchsorted() uses lexical order instead of the provided categorical order
Hi,
it seems that the searchsorted() method of Categorical series does not take into account the specific order of the categories.
#### An slightly modified example from the API documentation

``` python
x = pd.Categorical(
    ['cheese', 'apple', 'bread', 'bread',  'milk'],
    categories=['cheese', 'milk', 'apple', 'bread'],
    ordered=True)
print('Unsorted:', x)
x_sort = x.sort_values()
print('Sorted:', x_sort)
print('Searchsorted apple:', x_sort.searchsorted('apple'))
print('Searchsorted milk:', x_sort.searchsorted('milk'))
```
#### Output

Unsorted: [cheese, apple, bread, bread, milk]
Categories (4, object): [cheese < milk < apple < bread]
Sorted: [cheese, milk, apple, bread, bread]
Categories (4, object): [cheese < milk < apple < bread]
Searchsorted apple: [0]
Searchsorted milk: [5]

As you can see, "apple" is inserted at the beginning (b/c it starts with an 'a') and "milk" is inserted at the end, even though the order of both categories is between "cheese" and "bread". 
Unfortunately, the API documentation does not specify is this is the expected behavior, but it seems inconsistent with the fact that .sort_values() does use the categorical order. 
#### Output of `pd.show_versions()`

<details>
## INSTALLED VERSIONS

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-34-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0
nose: None
pip: 8.1.2
setuptools: 25.1.6
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.0
statsmodels: 0.6.1
xarray: None
IPython: 5.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.1
matplotlib: 1.5.1
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: None
httplib2: 0.9.2
apiclient: 1.5.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
