Identical to ELPA 2013.11.001 the build procedure is updated

  - updated INSTALL documentation
  - improved description of kernels src/elpa2_kernels/README_elpa2_kernels.txt
  - improved OpenMP builds:
      * usage of OpenMP can now be specified with
        "--enable-openmp" or "--with-openmp"
  - improved checks for AVX build (identical to ELPA 2013.08.003):
     * if AVX kernels are specified AVX compilation is tested
     * the option "--with-avx-optimization" automatically updates
       CFLAGS and CXXFLAGS with optimization flags