Scripts for building the full software stack for the Athena project in the master branch

This is a very simple version of the scripts meant to build AthenaExternals and Gaudi in an automated way as part of the build of the Athena project.

The scripts only require the user to set up an adequate version of GCC, CMake and git. All of which can be achieved like the following for instance:

    setupATLAS
    asetup none,gcc49 --cmakesetup
    lsetup git

After this setup the externals can be built in the default `RelWithDebInfo` mode by running:

    ./athena/Projects/Athena/build_externals.sh

Or if one wants to build in some other mode, it's possible to run for instance:

    ./athena/Projects/Athena/build_externals.sh -t Debug

With the externals built, the Athena project itself can be built in a relocatable way by executing:

    ./athena/Projects/Athena/build.sh

This script also accepts some options. Both scripts can be queried for the options that they accept with `-h`.

When the build is done, the results can be found in a directory called `build/install/`, and the produced RPMs are simply in the `build/` directory. Unfortunately the build has a _lot_ of issues at the moment. Which will need to be sorted out...

See merge request !165