ENH: Simplify using read_hdf for HDF files with one dataset
Allow read_hdf to be used without a key when a single pandas object
is stored in a HDF file.  Raises if multiple pandas objects found.

Small simplification to read_hdf when the HDF file is trivial.

``` python
df = pd.DataFrame([[1,2],[3,4]])
df.to_hdf('test.h5', 'df')
del df
df = pd.read_hdf('test.h5')
```
 minor doc point. ping when pushed/green.
 @jreback ready
 thanks!
