Pre-epoch dates giving OverflowError in mktime with HDFStore
I am having trouble with pre-epoch dates working with HDFStore, getting an OverflowError in mktime, as shown below.

Sample code is:

```
    import numpy as np
    from pandas import *


    def panda_test():

        # generate some data
        data = np.random.rand(50,5)    
        # generate some dates
        dates = DateRange('1/1/1969',periods=50)    
        # generate column headings
        cols = ['A','B','C','D','E']

        df = DataFrame(data,index=dates,columns=cols)

        # save to HDF5Store
        store = HDFStore('bugzilla.h5', mode='w')
        store['df'] = df # This gives: OverflowError: mktime argument out of range
        store.close()


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

The error I get is:

```
    Traceback (most recent call last):
      File "C:\Users\Salman Ansari\Documents\Python\VAR\src\bugzilla.py", line 27, in <module>
        panda_test()
      File "C:\Users\Salman Ansari\Documents\Python\VAR\src\bugzilla.py", line 22, in panda_test
        store['df'] = df
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 122, in __setitem__
        self.put(key, value)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 260, in put
        comp=compression)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 327, in _write_to_group
        wrapper(value)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 325, in <lambda>
        wrapper = lambda value: handler(group, value)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 335, in _write_frame
        self._write_block_manager(group, df._data)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 346, in _write_block_manager
        self._write_index(group, 'axis%d' % i, ax)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 432, in _write_index
        converted, kind, _ = _convert_index(index)
      File "C:\Python27\lib\site-packages\pandas-0.4.0-py2.7-win32.egg\pandas\io\pytables.py", line 698, in _convert_index
        for v in values], dtype=np.int64)
    OverflowError: mktime argument out of range
    Closing remaining open files: bugzilla.h5... done
```

Any ideas, please?
