CMake: fix versioning of dynamic avtranscoder library

Versioning in three steps:
     * libavtranscoder.so -> libavtranscoder.so.0
     * libavtranscoder.so.0 -> libavtranscoder.so.0.2.1
     * libavtranscoder.so.0.2.1
CMake: add version to avtranscoder-py dynamic library
CMake: add version to avtranscoder-java dynamic library
CMake: python file and its directory in lower case