created a copy of macos.h as basis for ios.h
add detection of Apple iOS (BOOST_OS_IOS)
improved detection of ARM platform (check for Apple clang __arm64);
set version to 8.0.0 in case of __arm64 (allows check for armv8 targets)
adjusted copyrights as requested by review
changed MacOS vs. iOS detection to match policies of library
added missing 'Copyright'
Merge pull request #2 from fdetro/os-ios

Add support for Apple iOS and improve detection of arm architecture. Thanks to Franz Detro for the changes.