removed default proxy port setting
Simplified proxy configuration array

proxy configuration array keys are now compatible with SoapClient,
i.e. only proxy_* keys are used, the short names ('host', 'port')
no longer allowed. Removed proxy_url, proxy_scheme and
http_auth_header; the url and header for the libxml stream are now
set in SchemaDocument constructor. proxy_port is now mandatory, the
default 8080 was removed.