added xmlrpc_encode_request wrapper
added htmlspecialchars

props to @cweiske https://github.com/pear2/Services_Pingback/blob/master/src/PEAR2/Services/Pingback/Client.php#L205
added missing htmlspecialchars
Merge pull request #3 from pfefferle/master

added a wrapper if xmlrpc is not installed
Adds support for rel=webmention (#5) and adds tests for other rel values.
adds support for rel=webmention in the http header as well