RFC 2976 INFO method. 

Dialog state is not modified, added sending API to nksip_uac.erl and a corresponding callback to nksip_sipapp.erl. Other than keeping the dialog's state, the handling is identical to BYE method. This can be usefull to somebody using nksip to implement a signalling gateway. I'm relatively new to both SIP and OTP so any comments will be greatly appreciated!