Changes to build RPMs, fixes issue #36

Added support for creating RPMs with shared libs that are installed in the
arch-dependent lib directory. To link tests/aseba-test-natives-count against
the shared libs I needed to define a handful of missing functions.
Added rpm/ directory with spec file and instructions for building RPMs.

Note that these changes are in the release-1.3.x branch because I'm assuming that is where the next release will come from. You should also merge the changes onto the master branch as you probably already do for other changes to the release-1.3.x branch. If you'd prefer a separate pull request for master, just let me know.