Improved documentation

* Improves documentation by re-arranging its structure and
  introducing full discussion about performance merit of
  intbitset.

* Overall, amends documentation to be compatible with
  reStructuredText.
  (closes #8)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Classifiers for Python 2 and 3

* Adds generic classifiers for Python 2 and 3, so that
  intbitset can e.g. show up in:
  <https://pypi.python.org/pypi?:action=browse&c=533>

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>