More options

This adds some easily supported 3.x options: DelayAttachOnConnect, IPv4Only, MaxMsgSize, ROUTERMandatory, TCPAcceptFilter, and XPUBVerbose.  Some were already supported but didn't have their own friendly methods yet.  I made a separate commit for each option, for easier review.

Tested on my machine with Go 1.0.3 and libzmq 2.2.0 & 3.2.2.

Does anybody find the casing of these a bit awkward?  Maybe gozmqgen is difficult for others to modify and needs more inline documentation?