Adding support for Windows Runtime (WinRT) to chrono.

Changes largely consist of updating Win32 APIs or disabling features which aren't possible.
Removing default building of Windows store, makes easier for contributing libraries individually.
Updating macro for detecting WinRT to BOOST_WINDOWS_RUNTIME based on code review feedback.
Updating checks for the Windows Runtime to use the correct Boost Predefs.
Reverting some whitespace changes that were unnecessary.
Merge branch 'develop' of https://github.com/boostorg/chrono into winrt
Merge pull request #3 from MSOpenTech/winrt

[winrt support] Enable Chrono to work in Windows store and phone 8