Yadis: drop custom replaceEntities() function and use html_entity_decode

Yadis' ParseHTML.php has a replaceEntities() function for replacing HTML entities, with a comment that appears to explain its existence by stating "Replace numeric entities because html_entity_decode doesn't do it for us". This is breaking with PHP 5.5 because it uses the deprecated /e modifier for preg_replace() - https://github.com/openid/php-openid/issues/108 . I think this custom function is no longer needed at least with PHP 5. PHP 5 has had support for replacing numeric entities since 2003, and its entity handling code has been refined quite a lot since then. replaceEntities() has been there since 2006, and probably earlier. I guess at that time PHP 4 compatibility was still important so this was needed - I don't think PHP 4's html_entity_decode() has ever had numeric entity support - but now PHP 5 is a decade old and there's a separate PHP 4 branch of php-openid, I think we can ditch replaceEntities() in the main branch!

It's a bit tricky running the test suite on a modern system, but I think I got it working well enough that the test that covers ParseHTML was working right. Without this patch it returns a bunch of 'EEEEEE' on my system (because of the preg_replace() /e modifier deprecation, I think); with this patch it runs without any 'EEE' type output, which I believe indicates that it passed. So I think that indicates the change is safe, but it'd be good if someone with more experience with php-openid's unit tests could verify.