Revising the HDF5 docs a bit, added another level to the toc
- makes the toc 4 levels
- increases the width of the side bar a bit
- increases the overall width just a bit

![io tools text csv hdf5 pandas 0 16 2 557 g23b9715 documentation](https://cloud.githubusercontent.com/assets/953992/9696820/5bbb9528-5346-11e5-8e52-15759d02257a.png)
![io tools text csv hdf5 pandas 0 16 2 557 g23b9715 documentation 1](https://cloud.githubusercontent.com/assets/953992/9696821/5d955190-5346-11e5-9e3f-3025c232f166.png)

@jorisvandenbossche 
 Looks good!

+1 on making it a bit wider (certainly for the side bar), but maybe 800 is enough (now the increase is quite big, from 720 to 840; 800 is what sphinx readthedocs theme uses)
