Default mach.bat to using py -2.

Servo already assumes the user has Python, this is the primary
way to make sure that Python 2 is preferred, and you should
get a sensible error message if you have python 3 but not 2.

But first, check that py.exe exists because if a system has only
Python 2.x only, it won't have it. If it doesn't, then try python.exe.

mach.bat

@@ -36,4 +36,9 @@ IF EXIST "%VS_VCVARS%" (
 
 popd
 
-python mach %*
+where /Q py.exe
+IF %ERRORLEVEL% NEQ 0 (
+  python mach %*
+) ELSE (
+  py -2 mach %*
+)