Use C++11 for the code base

Use C++11 for the code base

As the Enterprise Linux 6 platform has now essentially reached it's
end of life for what it's worth (the Fedora EPEL6 distribution is not
maintained anymore) nothing ties us to using C++03 only anymore.

So, I think it makes sense to move the code base to the C++11
standard.

Why C++11 and not, say, C++14 or more?  Well, the more direct reason I
see is that we need to support long life cycle platforms, the older
one being Enterprise Linux 7 currently.  This is the Fedora EPEL7
distribution, in concrete terms.  And in that distribution, the
compiler is GCC 4.8.x.  And it supports C++11.

In practise, nothing changes in the code that is already there.

The new code however can use C++11 constructs just fine.

I have updated the CONTRIBUTING file to write down some of the
unwritten cultural biases of the current code base.  Hopefully these
few lines will help to shed some light on the choices made so far.

The update to that file also enacts the use of C++11 and sets some
limits to what we expects in terms of what the code base would look
like.

configure.ac is modified to unconditionally pass -std=c++11 to the
compiler and express that in the configuration text displayed at the
end of the configuration stage.

Some Makefile.am files are updated accordingly.

	* CONTRIBUTING: Enact use of c++11. Also, we favor those who
	read/debug/maintain the code as opposed to those who write it ;-)
	* configure.ac: Switch to c++11 unconditionally.
	* src/Makefile.am: Adjust.
	* tests/Makefile.am: Adjust.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>