[winrt support] Replacing banned APIs Sleep and InitializeCriticalSection

Updated InitializeCriticalSection to InitializeCriticalSectionEx if on a new enough platform that supports.

Replaced Sleep in the yield function to use std::this_thread::yield if targeting the Windows Runtime.
The BOOST_PLAT_WINDOWS_RUNTIME macro is always going to be defined, that is how the Boost.Predefs work. It will just either have a true or false value.

http://www.boost.org/doc/libs/1_55_0/libs/predef/doc/html/predef/using_the_predefs.html