ENH: Iterate over HDF store hierarchically
I am relatively new to pandas, but have been using raw HDF5 files for a long itme, so apologies if this feature or something equivalent already exists.

As stated in the documentation, it is possible to create hierarchies of data stores:

``` python
store.put('/first_group/df1', df1)
store.put('/first_group/df2', df2)
store.put('/second_group/df3', df3)
```

However, it seems that `store.keys()` can only provide a flattened list of all available groups.  It would be nice to be able to walk the HDF5 file hierarchically.  A couple examples (this is borrowing a bit from the h5py API):

``` python
# do stuff on each subgroup/substore in /first_group
for s in store['first_group'].values() :
    # do stuff on df1 and df2

# visit each group at the root level
for s in store.values() :
    if 'df1' in s :
        # do stuff on any store named 'df1'
```

The main use case I would have for something like this is that I have many dataframes with different schema but would like to group some of them together and then iterate through those groups. 

Best way I can think to emulate this is to parse the strings in `store.keys()`, but that is pretty ugly.
