python: implement comparison operators

Comparison was previously implemented by means of cmp() methods. Now we
can do `f1 < f2` instead of `f1.cmp(f2) < 0`. Notably, "==" works as
expected. Related to #59.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>