Merge pull request #44 from stoewer/master

New unit test for nix::File

Tested locally. All fine.
Cleanup: Remove old Xcode project (use CMake instead)
Move old unit tests for Block
CMake: allow custom cxx flags using -DCMAKE_CXX_FLAGS_INIT
Add assignment operator for nullptr_t to nix::Block
Close and isOpen method for FileHDF5
Close and isOpen method for nix::File and IFile
Use the new close method in TestFile
Start unit test for Block