Raise an error if requiring rbreadline without ENV["HOME"]

No longer crash with a "can't add nil" error when importing rbreadline when ENV["HOME"], ENV["HOMEDRIVE"] and ENV["HOMEPATH"] are unset. This is a fairly common case when deploying a Rails app with something like upstart on ubuntu.

Now, at require time, it checks both to see if HOME is set and that it points to a directory, raising a comprehensible exception otherwise.

Fixes #8.