HDFStore to work with 'with' statements

This should work:

``` python
    with pandas.HDFStore('file.h5') as f:
        print(f)
```

as a more convenient form of 

``` python
    with contextlib.closing(pandas.HDFStore('file.h5')) as f:
         print(f)
```

AFAIK this is a easy as adding the following to `HDFStore`

``` python
    def _enter_(self): pass
    def _exit_(self): self.close()
```
