Remove superfluous includes from test file.
add missing include to osmium_index (for std::cerr)
fix double including for mmap_for_windows.hpp
Set CATCH_CONFIG_CPP11_NULLPTR in catch.hpp. Document changes to catch.hpp.

Also the original catch.hpp is kept around for comparing. This fix should make
the tests work on Windows, but it is a hack. This really must be fixed
upstream.