Fix for string literal parsing.

Use of "some" keyword meant string could not be empty, changed to "many"
keyword.
Closes #50.

src/Parser/Combinators/Lex.hs

@@ -225,7 +225,7 @@ stringLiteral =
    where stringLiteral' :: Char -> Parser String
          stringLiteral' container =
             char container
-            *> some (try $ validGraphicChars container)
+            *> many (try $ validGraphicChars container)
             <* char container
          validGraphicChars container =
             (string (replicate 2 container) *> return container)