Add missing package dependency declaration

cl-lib is not available in Emacs 23, but GNU ELPA carries a compatibility package. This commit ensures it will be installed if required when hamlet-mode is installed.
Add autoload cookies

With these lines present, users will not normally need to `require` hamlet-mode in order to use it.

It's idiomatic to modify `auto-mode-alist` after the mode declaration, so this commit moves the modification code lower in the file.
Note availability of MELPA package
Merge branch 'patch-1' of git://github.com/purcell/hamlet-mode

Adds a cl-lib dependency for compatibility.

Conflicts:
	hamlet-mode.el
Merge branch 'patch-2' of git://github.com/purcell/hamlet-mode

Conflicts:
	hamlet-mode.el
Merge branch 'patch-3' of git://github.com/purcell/hamlet-mode