The Visual Studio support has been totally revamped. Instead of using registry
magic, we use the .bat files available for each version of visual studio. This
is simpler (does not depend on the version of the compiler), more reliable, and
just plain better.

Fundamental changes
===================

env["ENV"] has been expanded a bit on windows:
	- add %SYSTEMROOT%\system32 in the path for windows
	- add COMSPEC (needed by the .bat file execution)

UI
==

For the user, there should not be any difference.  By default, the most recent
version of Visual Studio will be used, and the version can be tweaked by using
the MSVS_VERSION. A scons script can use different version of VS at the same
time.

Internals
=========

The code can be found in the MSVCCommon tool. At this point, no function should
be public, the exact API is not good yet. There are three parts:
        - finding the .bat file: this depends on the version, and the
          architecture. First, the registry is used, then the environment
          variable MS*COMMON. A default location could be looked for, too, but
          this is not done at the moment.
        - Parsing the .bat file: the .bat file is executed, and its output
          parsed. By Default, only LIB, LIBPATH, INCLUDE and PATH are taken.
        - Merge the found variables into env['ENV'].

Move msvc notes in our new module.
Move msvc notes in our new module.

