Update to 0.8.1

Changelog: highlights only
Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Fri Feb 7 00:55:18 2014 +0000

    pstream.h (basic_ipstream::basic_ipstream(argv_type, pmode): Use
    basic_ipstream::readable(pmode) to correctly set input mode.
    test_pstreams.cc: Test execve-style construction for ipstream.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Fri Jul 12 00:20:21 2013 +0100

    pstream.h: Add initializer-list constructors.
              Make unary constructors explicit.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Thu Jul 11 23:27:43 2013 +0100

    pstream.h: Add newpg pmode flag. Version 0.8.1.
    test_pstreams.cc: Test newpg flag.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Fri Mar 29 17:42:22 2013 +0000

    pstream.h: Doc tweak

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Wed Jan 23 00:43:11 2013 +0000

    pstream.h: Put child in new process group and define pstreambuf::killpg()

    Thanks to Hein-Pieter van Braam for the suggestion.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Sun Jan 20 19:15:41 2013 +0000

    pstream.h: Retry interrupted writes

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Sun Jan 20 17:11:27 2013 +0000

    pstream.h: Overload constructors for convenience.

    Bump version to 0.7.3

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Mon Jun 25 22:57:26 2012 +0100

    pstream.h: Update copyright years and remove RCSID.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Mon Jun 25 22:48:53 2012 +0100

    pstream.h (pstreambuf::xsputn): Optimize.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Sun Jun 24 13:03:43 2012 +0100

    pstream.h (basic_pstreambuf::open): Work with _FORTIFY_SOURCE.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Tue Nov 15 11:12:10 2011 +0000

    (basic_pstreambuf::wpipe, basic_pstreambuf::rpipe): Fix for clang.
    (PSTREAMS_VERSION): Bump to 0.7.2

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Thu Oct 14 19:57:41 2010 +0000

    * pstream.h, test_pstreams.cc: Update copyright dates.

Author: Jonathan Wakely <pstreams@kayari.org>
Date:  Thu Oct 14 19:55:19 2010 +0000

    * pstream.h (pstreams_common::pstreams_common): Use basic_ios::rdbuf
            to set the streambuf, basic_ios::init(0) has already been called.

(ryoon)