Feature cachebust

Do not merge.  A work in progress.
So the `_query` argument to `request.xxx_url()` can be an iterable of 2-tuples as well (since a key can be specified more than once in a query string). With this in mind perhaps you want to standardize it in some way prior to passing it in.

Arguably Pyramid should standardize it itself using a `MultiDict` but currently it does not so any code dealing with query strings is somewhat difficult to write.