Add Util.getCodeString

- stringify a function and its arguments.  Intended for passing a function
  object and its arguments into another execution context.  Implicitly
  handles any arguments that can be JSON.stringify'd.
Use module-level test descriptions

- consistent with specs for other modules
Return the exception's stack

- since the exception can't cross contexts, extract its text and return it
  instead of an Error object
- all other types are returned as-is
Simplify function+arg passing to background page
Update tester fcommand

- use simplified background-script passing scheme (function obj +
  stringifiable args)
- pull known parameters out of object passed to Fcommand