Use WebOb's multidict instead of Paste

Paste is obsolete these days: it has not been maintained in years. It code has been split up into multiple other packages that are actively maintained and widely used.

MultiDict has been moved to [WebOb[(http://webob.org/). It would be nice if flanker can switch to that (just change the import too `webob.multidict.MultiDict` so we can get rid of the paste dependency again.