Different HDFStores in multiple threads crashes Python
``` python
import threading
import pandas as pd
import time

def foo():
    store = pd.HDFStore('my_hdf_file.h5')
    store['foo']
    store.close()


def main():
    threading.Thread(target=foo).start()
    threading.Thread(target=foo).start()
    time.sleep(2)

if __name__ == '__main__':
    main()
```

Crashes for me (Windows 7 using pytables 2.4.0 and pandas 0.9.1 from http://www.lfd.uci.edu/~gohlke/pythonlibs/). I can't get the stack trace easily, I can try harder if necessary. Simply using tables.openFile and reading a few values seems to work fine.
