Gitignore: ignore CMake generated files
Travis build: use CMake
Travis build: remove install of python-git module
CMake: add cmake directory to have custom utilities

* Custom utilities are:
     * AvTranscoderMacros.cmake: macros, included in all cmake files.
     * FindFFmpeg.cmake: to get FFmpeg package (by specify a list of
components, check the FFmpeg version, get FFmpeg from command line if
there are parameters, etc...).