Major restructuring, untested

- Drop support for emacs 23 and below.
- Switch to setting "completing-read-function" (new variable in Emacs
  24) instead of defining advice on "completing-read".
- Rework old-style compatibility framework
- Version bump to 2.0
- Avoid spurious warning when ido-ubiquitous is enabled before ido during init
- Make ido-ubiquitous work in interactive forms of commands
Fix compile errors
Remove help text about interactive-mode problems.

This is fixed now, so the warning is no longer needed.
Add change log entries
Autoload fix
Update changelog
Doc changes
First working version after 2.0 refactor