Adds the SML language definition

Hey @isagalaev 

I took a moment to add the the language definition for Standard ML. I based the language in the definition of OCaml, but changed the keywords to follow the SML Language Specification as stated in the [SML Language Definition of 1997](http://mitpress.mit.edu/books/definition-standard-ml).

I tested the highlight features both in Node.js and the browser with dozens of code examples that I have created for a University course that I am taking and it looks pretty good. I think this is working fine.

It is probably that the language definition may still a few refinements here and there, I did spot minor problems, but this is also present in the OCaml file. Nothing that can be considered a big deal.

Well, that's all for the time being. I hope you accept my merge requests.

Cheers,
--Edwin
I took it from your snippet in #619, I though it was OCaml code (and also GitHub highlights `case` there). I now removed this keyword. Thanks!