MSVC installation wrapper script

Dave Page wrote:

Per previous discussion with Magnus, the attached batch file (rename to
install.bat) is a wrapper script for the MSVC installer. It sets up the
environment before installing (to allow zic.exe to run properly), and
removes the need to explicitly call perl on the command line.

This should be added to src/tools/msvc

Applied.

I took out the check for src\tools\msvc, since the script in general
doesn't work outside the msvc directory. (it needs to be in the same dir
as install.pl). We could fix that, but I don't really see the point, so
I kept it simple :-)

//Magnus