Refactoring and clean up elixir-mode

* clean up structure
* remove obsolete text.ex file
* use correct function prefixes
* autoload helper functions
* simplify version display
* add autoload elixir-mode for specific file endings.
* add documentation into elixir-mode.el file
* update README 
* add Rakefile with release task (pump version etc.)

/cc @antifuchs