Emphasise use of ctrl-d in the README.

Unfortunately the example has to be re-formatted into HTML to mark this up
properly.
Update install instructions for 0.4.
Clarify the setup steps in the contributing guide.
Bug fix: Don't output greeting on ctrl-c

Catching an `Interrupt` and `retry`ing the method was re-running too many
things, including the greeting and the history load.
Support Ruby 1.9.3

This should significantly simplify installation on Linux, since 1.9.3 is
available as a package for most major distros. Also, it paves the way for us
building Debian packages (etc.) that only depend on standard packages.
Do not insert blank lines into `Readline`'s history
Extract `ModuleDelegator` from classes that wrap `Readline`
Add ability to chaim multiple methods together.
Incremental code cleanup.