PERF: HDFStore iterator can use slices to represent iterator when where=None
from [SO](http://stackoverflow.com/questions/30587026/when-reading-huge-hdf5-file-with-pandas-read-hdf-why-do-i-still-get-memory)

```
In [2]: df = DataFrame(np.random.randn(1000,2),columns=list('AB'))

In [3]: df.to_hdf('test.h5','df',mode='w',format='table',data_columns=True)

In [4]: i = pd.read_hdf('test.h5','df',chunksize=10)

In [5]: i
Out[5]: <pandas.io.pytables.TableIterator at 0x108f397d0>

In [6]: i.coordinates
Out[6]: 
Int64Index([  0,   1,   2,   3,   4,   5,   6,   7,   8,   9, 
            ...
            990, 991, 992, 993, 994, 995, 996, 997, 998, 999], dtype='int64', length=1000)
```

These coordinates could be represented by `slice(0,1000,10)` in this case
