Strict mode error

Declaration of Redmine\Api\SimpleXMLElement::addChild() should be compatible with SimpleXMLElement::addChild($name, $value = NULL, $ns = NULL)
Update Group.php

Corrected bug in addUser. '/groups/'.$id.'/users.xml' instead of '/groups/'.$id.'/user/users.xml'. Tested and it works
fixed setPort if no port or scheme is detected
fix group - user tests
update readme + add phpunit to composer + remove un-needed docblocks from tests
cast port to int
Replaced PHP_EOL with "\n", fixes tests on windows [#75]