Windows Visual Studio port

This allows compiling 32bit MicroPython with Visual Studio 2013, with only minor changes to the code.
All tests pass except memoryerror.py due to #527.
The functionality is the same as for the unix port: file.c, gccollect.c and main.c have been reused, I still have to look into modtime.c and modsocket.c though because the differences with the Windows SDK are bigger.

Since there is so much reuse, is it maybe an option to make a more general 'PC' platform subdirectory so unix/windows/whatever ports that run on a PC can get their common files from there?
Or more general, maybe it's an idea to change the directory structure so there's a sinlge 'build' subdirectory with in turn subdirectories for each platform?