IntBitSet package

* Creates IntBitSet package.  Until now, intbitset was being
  developed within Invenio project, see <http://invenio-software.org/>.

* Commits prior to this one faithfully replicate commit history of
  intbitset inside Invenio, so that past development history can be
  easily tracked also here.  (Note that past commits are not directly
  buildable here uot of the box, as they require Invenio.)

* Adds support for Travis-CI.

* Adds initial Sphinx documentation.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>