Support Sylvan count()

[ricbit]

I can use the method count() on autoref and cudd, but it fails on sylvan. I suppose the count() was not mapped yet, the C interface seems to have it.

[johnyf]

Added dd.sylvan.BDD.count() on branch sylvan_count.

[slivingston]

@johnyf I am happy to review a PR with that branch.

[johnyf]

Thanks @slivingston, opened #105.