soap api: added wsdl-viewer

NuSoap offered a convenient, self-generated API documentation page. On
the other hand, the php-soap extension does not offer such functionality
and the wsdl's raw XML is not easy to read.

This commit adds Tomi Vanek's wsdl-viewer XSLT [1] to mantisconnect.wsdl
to offer a more user-friendly face to the XML.

[1] http://tomi.vanek.sk/index.php?page=wsdl-viewer