Revert "distinguish between options and defaults"

This reverts commit e7804c5027633d18558c05b550ea11c46fb37c42.
Revert "replace macro constants with enum types for user facing constants"

This reverts commit d4bb6d693f0542bbf35bc1b6810100d5b2a901a8.
Merge pull request #703 from hintjens/master

Reverted use of enums for zmq.h constants (too painful for binding authors)
Update NEWS
Identity can only be set on sockets that can connect to ROUTER
Merge pull request #705 from hintjens/master

Fixed doc for IDENTITY socket option
Rename engine's methods to improve code readability
Merge pull request #706 from hurtonm/master

Rename engine's methods to improve code readability