Include TCP_ACCEPT_FILTER for 3.x.
Add SetSockOptStringNil method on Socket.

Since Go will not allow nil where a string is expected, there is no way
to change the implementation of existing methods to accept nil strings.
This is necessary to properly support e.g. TCP_ACCEPT_FILTER, which can
only be cleared by setting it to nil.
Add a test for SetSockOptNil.
Retrieve errno() in a thread-safe way.

http://code.google.com/p/go/issues/detail?id=709#c3 shows how errno()
can be checked in a thread-safe way with a familiar idiom.

https://zeromq.jira.com/browse/LIBZMQ-338 shows the libzmq devs aren't
sure zmq_errno() should be used at all.  The libzmq source as far back
as v2.1.0 includes a one-line zmq_errno() implementation that only
returns errno, adding nothing to errno().
Merge pull request #54 from jtacoma/threadsafe-errno

Retrieve errno() in a thread-safe way.
Merge pull request #53 from jtacoma/tcp-accept-filter

Support TCP_ACCEPT_FILTER in 3.x.
Fix bug in (s *zmqSocket) Recv(flags SendRecvOption)
Use pkg-config, instead of assuming where libs and headers are.
Change the timeout argumeny in Poll() from uint64 to time.Duration.
Make it work for both ZeroMQ versions 2 and 3. The unit for
timeout changed between these two versions from microsecond
to millisecond.
Merge pull request #56 from pebbe/RecvFix

Fix bug in (s *zmqSocket) Recv(flags SendRecvOption)
Merge pull request #57 from pebbe/pkg-config

Use pkg-config, instead of assuming where libs and headers are.
Merge pull request #58 from pebbe/Poll-timeout-ZHQ3

Change the timeout argumeny in Poll() from uint64 to time.Duration.
Fix bug in SetSockOptStringNil.

Update README from godocdown.
Fix symbol error in 3.x tests. Does not complete however.
Update AUTHORS.
Correct RCVHWM value for zmq 3.x.
Correct SNDMORE type for zmq 3.x.

The type for zmq 2.x is actually also wrong, but uint64_t vs. int64_t
doesn't matter since it's a boolean value.
Poll supports -1 timeout.
Merge pull request #63 from somia/master

More 3.2 fixes.
Remove destroy() methods.

Go does not automatically set destroy() methods as finalizers and no
code in gozmq calls runtime.SetFinalizer, nor does any code in gozmq
ever call one of the destroy() methods.