unittests: Introduce GoogleTest framework usage

Prepare infrastructure to make unit tests using GoogleTest.
Add build files to gitignore

It's nice to build the project and have nothing listed as untracked
with 'git status'. This will reduce the chances of accidentally
committing binary, configuration, or build files.
Fix debian package building mechanism

This comit makes it possible to build debian packages using
dpkg-buildpackage. This fix is a kind of workaround -- it makes
it possible to compile and install the sources using the same
commands as when using autotools, but actually uses cmake build
system. It is still experimental, ie. not tested very well.