posix_mq 0.5.0

Uncommonly raised exceptions due to programmer error are now
raised more safely with respect to the MRI garbage collector.

The "posix-mq.rb" executable has been renamed to "posix-mq-rb"
to respect RPS and is also hopefully easier to type/read when
used in scripts.  I'm preserving the "rb" in the name since I
hope to have a more portable, C-only version someday.

Minor cleanups and documentation now uses RDoc 2.5.x