Improve compatibility with singledispatch

Discussed in https://github.com/mrocklin/multipledispatch/issues/23
No specific process in place.  I would probably use `DeprecationWarning`, although I'm also happy with any process you'd like to impose. 

In general I haven't gotten the sense that `multipledispatch` is widely used outside of projects that I manage personally.  As a result I don't mind being a bit sloppy with some of these processes.  I'm currently more concerned about a high-quality result than the smoothness of the path to that result.