Order of magnitude improvement on HDF5 parser speed

Fixes #529 

@josenavas and @adamrp, can you review please?

Former parsing:

```python
> %timeit t = load_table('../American-Gut/data/AG/AG.biom')
1 loops, best of 3: 4.91 s per loop
```

New parsing:

```python
> %timeit t = load_table('../American-Gut/data/AG/AG.biom')
1 loops, best of 3: 383 ms per loop
```
should `iteritems` be used here?