Fix a few inaccuracies in readme.
Add support for compiling using Visual Studio 2012 Express.
Modify cake.registry to use its own WOW64 constants.

This lets it work even if the 'winreg' module doesn't
support these constants and means we dont' have to print
out a warning message on 64-bit Windows.
Improvements to make Cake work with Visual Studio WDExpress 2012.

- Enumerate all installed Windows SDK directories and try these,
  latest version first, if the compiler doesn't have its own
  PlatformSDK directory.
- Only use a particular Windows SDK version if it contains
  libraries for the desired target architecture.
- Try using x86_amd64 cross-compiler if native amd64 compiler
  is not present. This is the case for WDExpress.