Fix use of flet for Emacs > 24.1, and enable assert.
Use define-derived-mode

This allows to inherit from `prog-mode', which is very convenient. It also
simplifies the declaration, and do not recreate the syntax table on each
lua-mode call.

Signed-off-by: Julien Danjou <julien@danjou.info>
Avoid polluting "global namespace" when striving for backward compatibility
Merge pull request #39
Add backward compatibility aliases for cl-assert & cl-labels functions
Fix bug with lua-mode-syntax-table initialization
Extend and unify backward compatibility code
Fix population of lua-mode-syntax-table (issue #42)

Previously, `modify-syntax-entry' calls were altering syntax table in use
when loading lua-mode.el (kudos to github/ryuslash for finding out).
Update README.md

Update installation section a bit