Unify default serializer definition
No registered functions test fails

Must specify a server for the worker first.
Ensure exceptions are enabled for all servers
Send JSON-serializable exception data
Use JSON by default, not pickle
Simplify TestException
Add note on serializer change
Change internal serializer name

Calling the serializer pickle is potentially misleading.
Break out testing configuration
Version 0.4
Merge branch 'v0.4-dev'