WIP-Improved documentation

* Improves documentation by re-arranging its structure and
  introducing full discussion about performance merit of
  intbitset.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
mmh... if using fastdump/fastload that is independent of Python version. If using pickle protocol than that's also guaranteed to work with different Python versions... Or did you mean something else?