Added autoload comment for MELPA

Using this, now MELPA generates an autoload declaration so that the file is loaded when you C-c C-c an SML code block in an org file.
Strip '-' printed by SML/NJ when evaling code blocks with leading empty lines

See this issue for example: https://github.com/swannodette/ob-sml/issues/3
Merge pull request #4 from spacemanaki/patch-1

Added autoload comment for MELPA
Merge pull request #5 from spacemanaki/master

Strip '-' printed by SML/NJ when evaling code blocks with leading empty lines