jsonrpclib is now a mandatory requirement
Update version number to 0.2.1
Version number updated to 0.2.1
Removed utils.StringTypes + code review

- Removed utils.StringTypes constants, replaced by utils.string_types
- Added documentation to classes and methods missing it
- Updated version number to 0.2.1
Better lookup of the custom serializer to look for

Using isinstance(types) before dict[type] wasn't a sufficient test:
an object of a child type could pass the first test, but would result in
a KeyError using the dictionary.