Various fixes and improvements

* Use standard header/footer lines for compatibility with package.el
* Include auto-mode-alist entry by default
* Use a proper mode definition rather than a plain function
* Support comments via the syntax table, not font lock keywords
* Use define-abbrev-table where appropriate
* Specify keywords more concisely, and pass "case-fold" flag with font-lock defaults to allow case insensitivity

This is all in connection with `dockerfile-mode`'s addition to [MELPA](http://melpa.milkbox.net/) today. I'm a tinkerer, and couldn't resist fixing this up a little along the way.

Cheers,

-Steve