BUG: memory leak in unique() with object dtype
#### Code Sample, a copy-pastable example if possible

The following consumes an increasing amount of memory, and eventually dies.

```python
import gc
import pandas as pd
import numpy as np

x = pd.Series(np.arange(10**6) % 10**3).astype(str)
while True:
    x.unique()
    gc.collect()
```

#### Problem description

When calling `unique()` on a series of objects, memory is allocated which cannot be freed from Python. This affects the current GitHub version of Pandas.

#### Expected Output

Code should run forever with stable memory usage.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.8-100.fc24.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C
LOCALE: None.None

pandas: 0.19.0+824.gf114af0
pytest: 3.0.5
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
xarray: 0.9.1
IPython: 4.2.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: 0.999
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
