Remove more outdated keywords, add some new ones.
Set tab widths for elixir files to the basic offset.
Update README a bit

Update the Bugs section, add a note about ruby-end-mode
Reorganize everything: Autoloads, requires, reindent elixir-mode.el

* Move the require for elixir-smie to the top of the file
* Move syntax table definition over to elixir-smie
* Add auto-updated autoloads file, elixir-mode-setup.el, which
  contains (elixir-mode-setup), the function responsible for
  associating file extensions with elixir-mode.
* Update URLs to point to the new home of this mode -
  https://github.com/elixir-lang/emacs-elixir and to the (old) new
  home of elixir - elixir-lang.org.
Update README for new setup instructions.