Add Unionizer

This pr addes the unionizer implementation which will take a layers-worth of features and union the 'like' features together into single geometries. There are a bunch of parameters to tweak which are explained in the conf parsing bits. I should add this info to the readme (look for a another commit to this pr). I should also note that this is not tested. I just wanted to get this pr out there for review while I write the tests and documentation. So yeah again, look for more commits.
A possible solution: Make geometries with zero total length non-unionable (as in `bool unionable(const mapnik::feature_ptr& feature, const set<string>& tags)` returns false)? Sounds like a good unit test to have, anyway :smile:

And we could, probably should, have a check for degenerate geometries between all izer phases. Or at least one which we can turn on in debug mode.