Fix crash on startup under Windows

Don't assume that the runtime initializes the __wargv global variable.
Remove OS X-specific instructions from README.md

They don't really belong here and are already on the linked website.
Don't set the background color under Windows

... unless the current background color is too bright.
Fix flickering progress bar under Windows
Fix restoring original console color under Windows
Add changelog entry for 1.4-windows-r1