Replace HTML entities with Unicode characters

As per [Derhuerst's suggestion](https://github.com/IQAndreas/markdown-
licenses/pull/2) replace HTML entities (such as `&ldquo;` and `&rdquo;`)
with actual Unicode characters (such as `“` and `”`). This is because
they should show up just fine anyway, and it makes reading the plain-
text source of the markdown file easier.
Fix outdated URL in README