Initial package release

* Creates intbitset package.  Until now, intbitset was being
  developed within the Invenio project main source code base.
  See <http://invenio-software.org/> for more information.

* Adds support for Travis-CI.

* Adds initial Sphinx documentation.

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

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