Making custom_pip_install.sh Windows friendly.

Towards #1123.

scripts/custom_pip_install.cmd

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+# Copyright 2015 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+:<<"::batch"
+@echo off
+pip install --upgrade %*
+goto :end
+::batch
+
+cd $(dirname $0)
+./custom_pip_install.sh "$@"
+exit $?
+
+:<<"::done"
+:end
+::done

tox.ini

@@ -4,7 +4,7 @@ envlist =
 
 [testenv]
 install_command =
-    {toxinidir}/scripts/custom_pip_install.sh {opts} {packages}
+    {toxinidir}/scripts/custom_pip_install.cmd {opts} {packages}
 commands =
     nosetests
 deps =