Bump CMake version requirement to 2.8.9

Time to modernize our build system, making use of all the fancy features
that have crept into CMake in the past few years. While 2.8.9 is by far not
the latest version, it is rather widely deployed in distros at this time and
thus seems to be a reasonable choice as a minimum requirement.
Remove obsolete CMake scripts

We no longer need to bundle our own versions of FindOpenSSL and
FindPkgConfig, as recent CMake versions ship them.
Update currently bundled CMake scripts

We still need to bundle some CMake scripts that are not part of a
standard CMake installation, and may not be present in particular
on non-UNIXy platforms (as e.g. Windows does not have the concept
of a system-wide CMake script location).

Let's use current versions of those.
Update README and COPYING-CMAKE-SCRIPTS

Just to make clear how to handle the files we bundle.
genversion-B-GONE

Back in the day, we wanted to have information about the git hash a
particular Quassel binary was built from in the About dialog, to allow
for diagnostic purposes. However, we didn't manage to retrieve that
kind of information using CMake alone (we're talking CMake 2.4 here, too);
so we came up with the hack to compile a small binary that would be run
at build time and called the git executable in order to write version
information into a file.

This is not particularly nice, as running random binaries at build time is
frowned upon and can cause problems in some setups, and of course we have
the overhead of building it first.

Meanwhile, we found a nice set of CMake macros created by Ryan Pavlik that
would retrieve that information purely running Git commands, making genversion
obsolete. Together with the configure_file feature of CMake, this allows us
to do this more elegantly.

So gone is genversion, and also the good old trusted version.inc file...
Don't look recursively for a .git dir

The original GetGitRevisionDescription.cmake script would look for
a .git dir recursively in the parent directories of its invocation;
while a nice feature, it may pick up an unrelated repository if there's
none in the Quassel directory. Because we know exactly where to find
our own .git if it's there at all, we can remove the recursion and
prevent this issue.