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.
Add Context.Set() and Context.Get() for options.

In 2.x this requires delaying zmq_init until the first socket creation.