Cleanup CmakeLists and require 2.8.5+

After this pull-request you will need at least **cmake 2.8.5+** [see here](http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=33f5a83a5d7aa4c5a021c33a3fa55069da5eeed8).

### Detailed Changes:
- rename deprecated variables:
  - MPI_INCLUDE_PATH -> MPI_C_INCLUDE_PATH
  - MPI_LIBRARIES -> MPI_C_LIBRARIES
  - HDF5_INCLUDE_DIR -> HDF5_INCLUDE_DIRS
- System Libraries:
  - use [SYSTEM](http://cmake.org/cmake/help/v2.8.11/cmake.html#command:include_directories) property to suppress warnings in external header files (nvcc does not do this [yet](http://www.cmake.org/Bug/view.php?id=14415))
  - LINK_DIRECTORIES is NOT necessary, since FIND_PACKAGE always returns an
    [absolute path](http://cmake.org/cmake/help/v2.8.11/cmake.html#command:link_directories)
- fix comment for CUDA_MATH
- rename and invert variable DISBALE_COMPILER_FLAGS_IN_SUBPROJECTS to
  SAME_NVCC_FLAGS_IN_SUBPROJECTS
- hdf5/libSplash
  - remove PIC_ENABLE_HDF5 Option and use HDF5_FOUND
  - check for splash.h and libsplash.a (update your libSplash installations!)
- png output
  - remove PIC_ENABLE_PNG and see if we can find libPNGwriter
  - add freetype finder