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 history can be tracked.

* 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>