fix python3 compat
Make Serializers Dumber

Let's have the serializer just worry about serializing the dict
that we hand it, and move the unicode stuff up to a serialize module.

This should hopefully let us move toward using a version string in
cassettes.
fix a couple imports for py3
update serializers
update old cassette detection
flickr test was actually using yaml cassettes
not really very happy about this
encoooode
Just make all the headers lower for now