Add autoloads for rhtml-mode and auto-mode-alist

This allows RHTML to work for package management users without needing
to add (require 'rhtml-mode) to their init.el.

Signed-off-by: Erik Charlebois <erikcharlebois@gmail.com>
Merge pull request #7 from erikcharlebois/master

Add autoloads for rhtml-mode and auto-mode-alist