Factorize fails with read-only array
same as in #15286 


Factorize raises a cython error when used with a read-only array. Seems related to #10043 and #10070. I discovered this via xarray via pydata/xarray#818.
#### Code Sample, a copy-pastable example if possible

``` python
a = np.arange(2)
a.flags.writeable = False
pd.factorize(a)
```

Raises the following

```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-54-dddd925d767a> in <module>()
      1 a = np.arange(10)
      2 a.flags.writeable = False
----> 3 pd.factorize(a)

/Users/rpa/anaconda/lib/python2.7/site-packages/pandas/core/algorithms.pyc in factorize(values, sort, order, na_sentinel, size_hint)
    194     table = hash_klass(size_hint or len(vals))
    195     uniques = vec_klass()
--> 196     labels = table.get_labels(vals, uniques, 0, na_sentinel, True)
    197 
    198     labels = com._ensure_platform_int(labels)

pandas/hashtable.pyx in pandas.hashtable.Int64HashTable.get_labels (pandas/hashtable.c:7893)()

/Users/rpa/anaconda/lib/python2.7/site-packages/pandas/hashtable.so in View.MemoryView.memoryview_cwrapper (pandas/hashtable.c:29882)()

/Users/rpa/anaconda/lib/python2.7/site-packages/pandas/hashtable.so in View.MemoryView.memoryview.__cinit__ (pandas/hashtable.c:26251)()

ValueError: buffer source array is read-only
```
#### Expected Output

Should be the same as with a non-read-only array

``` python
>>> pd.factorize(np.arange(2))
(array([0, 1]), array([0, 1]))
```
#### output of `pd.show_versions()`

commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.0
setuptools: 20.2.2
Cython: 0.22.1
numpy: 1.10.4
scipy: 0.16.0
statsmodels: 0.6.1
xarray: 0.7.2-4-g33efdcd
IPython: 4.0.0
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.5.0
pytz: 2016.1
blosc: None
bottleneck: 1.0.0
tables: 3.2.1.1
numexpr: 2.5.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 1.0.0
xlsxwriter: 0.7.3
lxml: 3.4.4
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.5
pymysql: None
psycopg2: 2.6 (dt dec pq3 ext)
jinja2: 2.8
boto: 2.38.0
