ENH: Read entire group of an HDF5 file in a single call
See this [question in SO](http://stackoverflow.com/questions/22897597/read-entire-group-in-an-hdf5-file-using-a-pandas-hdfstore).

I guess that something like `store.get_group('df')` might do the job:

```
>>> import pandas as pd
>>> store = pd.HDFStore('test.h5',mode='w')
>>> store.append('df/foo1',DataFrame(np.random.randn(10,2)))
>>> store.append('df/foo2',DataFrame(np.random.randn(10,2)))
>>> store.get_group('df')
...           0         1
... 0 -0.495847 -1.449251
... 1 -0.494721  1.572560
... 2  1.219985  0.280878
... 3 -0.419651  1.975562
... 4 -0.489689 -2.712342
... 5 -0.022466 -0.238129
... 6 -1.195269 -0.028390
... 7 -0.192648  1.220730
... 8  1.331892  0.950508
... 9 -0.790354 -0.743006
... 0 -0.761820  0.847983
... 1 -0.126829  1.304889
... 2  0.667949 -1.481652
... 3  0.030162 -0.111911
... 4 -0.433762 -0.596412
... 5 -1.110968  0.411241
... 6 -0.428930  0.086527
... 7 -0.866701 -1.286884
... 8 -0.649420  0.227999
... 9 -0.100669 -0.205232
... 
... [20 rows x 2 columns]
```

I never did a pull request before, nor tried to dive into pandas code, but I guess that a possible implementation (inspired in [this answer](http://stackoverflow.com/a/22897898/630598)) would be something like:

```
class HDFStore():
    ...
    def get_group(self, group_name):
        return pd.concat([ self.select(node._v_pathname) for node in self.get_node(group_name) ])
    ...
```
