Add Windows installer creation script, mknsis.sh.

mknsis.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+PATH=$PATH:/usr/i686-pc-mingw32/sys-root/mingw/bin	\
+	nsiswrapper --run \
+		--name "CPU miner" \
+		--outfile cpuminer-installer.exe \
+		minerd.exe