CMake: build AvTranscoder

* Same problem with scons about the java binding : need to generate the
java files, reconfigure, and build again to generate class files and the
jar. Need to find a solution to skip the reconfigure...
* Useful flags to configure using CMake:
     * CMAKE_BUILD_TYPE: Release / Debug
     * CMAKE_INSTALL_PREFIX: /path/to/install
     * FFMPEG_INCLUDE_DIR
     * FFMPEG_LIBRARY_DIR
* Name of output librairies:
     * avtranscoder.a
     * avtranscoder.so (link + .so with version)
     * avtranscoder-py.so (without link)
     * avtranscoder-java.so (without link)
CMake: build cpp apps

* Add a CMakeList.txt for each app.
CMake: fix install of libraries

Do not need to reconfigure the make file to be able to install
libraries.
CMake: fix java binding

* Do not need to reconfigure the make file to be able to generate class
files and jar file.
* The solution was to add a custom command as a post build of the java
binding which generate java files.
CMake: fix build with dependency to libav

Check if FFmpeg or libav dependency, and define variable to manage
includes in source code.
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...).
CMake: get swresample or avresample

* Update FindFFmpeg.cmake : do not raise error if a component is not
found (warning).
FindFFmpeg.cmake: manages REQUIRED statement

* If REQUIRED is specified for modules, raise error if a module is not
found.
FindFFmpeg.cmake: set FFMPEG_FOUND if FFmpeg is found on the system
CMake: clean find java package

* No need to specified the jdk version (it could be different on an
other computer).
* Use the JAVA_HOME environment variable to get the jdk (make sure your
env. is correctly set).
Travis: add install libxmu-dev

* Library linked by target "avplay".
Merge branch 'master' of https://github.com/avTranscoder/avTranscoder into build_CMake
CMake: define preprocessor macro AVTRANSCODER_VERSION_... through cmake

* Define preprocessor macros:
     * AVTRANSCODER_VERSION_MAJOR.
     * AVTRANSCODER_VERSION_MINOR.
     * AVTRANSCODER_VERSION_MICRO.
* In CMake AVTRANSCODER_VERSION is available.
CMake: clean javac and jar command

* Get javac executable from Java_JAVAC_EXECUTABLE.
* Get jar executable from Java_JAR_EXECUTABLE.
Travis: fix build
FindFFmpeg.cmake: add comments
CMake: no compiler chosen

It could be g++, clang, msvc...