posix_mq 0.3.0

This release adds a few new API methods, fixes MRI 1.8.6
support.  We should now have full feature parity with
underlying POSIX message queue C API.

* POSIX_MQ#notify(&block)
  RDoc: http://bogomips.org/ruby_posix_mq/POSIX_MQ.html#M000001
  This is only supported on platforms that implement
  SIGEV_THREAD with mq_notify(3) (tested with glibc + Linux).
  Other platforms will have to continue to rely on signal
  notifications via POSIX#notify=signal, or IO notifications
  in FreeBSD (and Linux).

* POSIX_MQ#shift([buffer [,timeout]])
  Shorthand for the common "POSIX_MQ#receive.first"
  when you do not care for priority of the received message.

Rev, EventMachine and Reactor support are planned for
Linux, FreeBSD and possibly any other platforms where POSIX
message queues are implemented with a file descriptor.