runtime/ref, runtime/base: Fixes for _whenMoreResolved() support.

The yelling example now completes without errors. It still doesn't work
though.
runtime/data: Implement String.join().

Does type inspection to decide whether joined items are Strings or Characters.
Suggestions for improvement are welcome, but this is the only way to turn
Characters into Strings that I know of.
runtime/ref: Make horrible patchouts because I don't grok Refs.

@washort, I could use some help or advice here.
runtime/scope, runtime/trace: Add traceback() primitive.

For debugging only. Prints the traceback of the most recent Python-level
exception. Supremely useful.
examples/yelling: Massage incoming bytes into a string.

And now it works! Well, kind of. It only pumps 16 bytes at a time, regardless
of how much data's actually pending. And it doesn't respect EOF. We should use
Twisted for all of this.