UnboundLocalError: local variable 'store' referenced before assignment
in ensure_clean_store on ubuntu 13.10 triggered by many tests, e.g.

```
======================================================================
ERROR: test_unicode_index (pandas.io.tests.test_pytables.TestHDFStore)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 4506, in test_unicode_index
    compat_assert_produces_warning(PerformanceWarning,f)
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 121, in compat_assert_produces_warning
    f()
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 4504, in f
    self._check_roundtrip(s, tm.assert_series_equal)
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 4122, in _check_roundtrip
    with ensure_clean_store(self.path, 'w', **options) as store:
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 77, in ensure_clean_store
    safe_close(store)
UnboundLocalError: local variable 'store' referenced before assignment

======================================================================
ERROR: test_unimplemented_dtypes_table_columns (pandas.io.tests.test_pytables.TestHDFStore)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 1949, in test_unimplemented_dtypes_table
_columns
    with ensure_clean_store(self.path) as store:
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 77, in ensure_clean_store
    safe_close(store)
UnboundLocalError: local variable 'store' referenced before assignment

======================================================================
ERROR: test_versioning (pandas.io.tests.test_pytables.TestHDFStore)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 416, in test_versioning
    with ensure_clean_store(self.path) as store:
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/tmp/buildd/pandas-0.15.0/debian/tmp/usr/lib/python2.7/dist-packages/pandas/io/tests/test_pytables.py", line 77, in ensure_clean_store
    safe_close(store)
UnboundLocalError: local variable 'store' referenced before assignment
```

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.0-4-amd64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C

pandas: 0.15.0
nose: 1.3.0
Cython: 0.19
numpy: 1.7.1
scipy: 0.12.0
statsmodels: 0.5.0
IPython: None
sphinx: 1.1.3
patsy: 0.3.0
dateutil: 1.5
pytz: 2012c
bottleneck: None
tables: 2.4.0
numexpr: 2.0.1
matplotlib: 1.2.1
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.4
xlsxwriter: None
lxml: None
bs4: 4.2.0
html5lib: 0.95-dev
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
