Added batch script to cleanup any build stuff.

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@290 67173fc5-114c-0410-ac8e-9d2fd5bffc1f

clean.bat

@@ -0,0 +1,6 @@
+cd code
+mingw32-make -f makefile.mingw clean
+
+cd ..
+del /Q /S obj bin
+