Improve usage syntax by making subscripts return JSONValue instead of Optional, avoiding all the ?'s.
Merge pull request #6 from JJJayway/improveSubscriptSyntax

Dictionaries and arrays now return JSONValue instead of JSONValue?. This makes the code clearer, and after more thought, I do not think this breaks the semantics of Swift as we are still constraining values to enums, which is arguably more correct.