API: HDFStore closing is buggy
Testing and playing with HDFStores a lot, I have found several times that a store.close() leaves things hanging, like this:

``` python
store.close()
# no output
store
ClosedFileError: the file object is closed
df.to_hdf('test.h5','df',data_columns=['A','B'],mode='w',table=True)
ValueError: The file 'test.h5' is already opened.  Please close it before reopening in write mode.
# The store was opened with 'test.h5' as filename.
```

pandas version 0.12.0
