Windows platform fixes

Windows platform fixes, including:

 - set application icon
 - sonic-pi.bat for startup
 - rp-build-windows.bat file to build and copy dependencies
 - start ruby.exe from PATH
 - search for scsynth.exe in both Program Files directories
 - fix scsynthexternal.rb boot process to run scsynth.exe in the background, and not die if recvfrom fails
 - show messages on startup errors: no ruby, server startup failed/timed out
 - don't use git if Rugged isn't present (future: use Grit instead)
 - Windows build instructions in INSTALL.md