Normalize channel names in window.irc.IRC.

They are always keyed by lower-case key, but many parts of external
code were arbitrarily accessing elements of the "channels" object
without case-folding. Make it an internal-only "_channels" object
wrapped by accessors for find/new/delete which manage the object's
internals.

Resolves issue #151, and related problems of channel matching.

[Not as much Coffee cleanup in this as I'd like to do, but didn't want to make the change look more complicated than it already is.]