python: Add support for python3+

Added a --with-python=VERSION option to configure.ac, and changed the
python destination to work with both python2 and python3. Tested with
python 2.7 and 3.3.

The configure check defaults to searching for the unversioned "python"
package.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>