Add Transport:secure/0

Currently Ranch checks if a connection is secure by checking if its name
is 'ssl'. This isn't a very modular solution, adding an API function
that returns whether a connection is secure.
Don't report error on ssl {error, closed}

SSL socket might be closed on accept_ack, it happens quite often
and it is not a problem, so don't report error on the case.
Merge branch 'ssl_closed' of git://github.com/yjh0502/ranch
Merge branch 'add_transport_secure' of git://github.com/matrixise/ranch