Emacs 24.4 support and a workaround for issue #7

This pull request adds support for Emacs 24.4. Tested with Emacs 24.4.51.1 and 24.3. It also disables the slime-repl-mode integration due to issue #7.

Using lexical-binding turned out to be the best way to avoid warnings from the compatibility code and, anyway, the 70s called, they say lexical binding is the future! :-)