Fix #57 -- SmartyPants handling of single quotes.

`git bisect` indicates that the single quote bug was introduced in commit ad2c7fe1b05a6af47ed8a8a7091808e8d8140697.  The problem is that by the time quotation marks are being converted, `'` has already been changed into `&#39;`, and the code to convert that into a smart quote is only looking for `'`, not `&#39`.

There was already code to handle the fact that `"` had been converted into `&quot;`, but it appears that single-quotes were overlooked.