Session API: fix links to /api/0.6.0/sipMessage/
Session#refer API: add missing parenthesis

Thanks to SD for bringing this to light:
https://groups.google.com/d/msg/sip_js/Fj7y396a92I/jkNNnN0U5vIJ
Session#refer API: add followRefer callback arguments/types

Thanks to SD for bringing this to light:
https://groups.google.com/d/msg/sip_js/Fj7y396a92I/jkNNnN0U5vIJ