DOC: cookbook for in-memory HDFStores usage
When writing a DataFrame to hdf format ('to_hdf' method) you cannot pass a file handle instead of a string. i.e. you can't:

``` python
with open('test.hdf') as f:
    df.to_hdf(f, "df")
```

This raises an error:

```
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "C:\Anaconda\lib\site-packages\pandas\core\generic.py", line 896, in to_hdf
    return pytables.to_hdf(path_or_buf, key, self, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\io\pytables.py", line 286, in to_hdf
    f(path_or_buf)
  File "C:\Anaconda\lib\site-packages\pandas\io\pytables.py", line 279, in <lambda>
    f = lambda store: store.put(key, value, **kwargs)
AttributeError: 'file' object has no attribute 'put'
```

This is inconsistent with the 'to_csv' method where you can do this. In the docs it states that you can pass a file buffer to both 'to_hdf' and 'to_csv'.

pd.show_versions(False)

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.5.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 45 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.14.1
nose: 1.3.4
Cython: 0.22
numpy: 1.9.2
scipy: 0.15.1
statsmodels: None
IPython: 3.0.0
sphinx: 1.2.3
patsy: 0.3.0
scikits.timeseries: None
dateutil: 2.4.1
pytz: 2015.2
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: 4.3.2
html5lib: None
httplib2: 0.9
apiclient: None
rpy2: 2.5.6
sqlalchemy: 0.9.9
pymysql: None
psycopg2: None
```
