Cabalify Alex and Happy

There's no need to invoke alex and happy from a Makefile. In fact,
invoking them via the Cabal file makes it easier for people to build
Hython out-of-the-box.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Merge pull request #2 from penberg/cabalify-alex-and-happy

Cabalify Alex and Happy