[autotest] Use gmail API to send emails.

Add a library that can be used by other code to send emails via
gmail api.

It can also run as a standalone script.

TEST=Ran
./site_utils/gmail_lib.py -s 2012-dut-board-inventory
fdeng@google.com fdeng@chromium.org <
/usr/local/autotest/logs/boards-2015-02-17.16.txt
BUG=chromium:465632,chromium:468622

Change-Id: Ie59986f4f8ef68fd1c2e7f62f4ec6b50e07f49bb
Reviewed-on: https://chromium-review.googlesource.com/261074
Reviewed-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Fang Deng <fdeng@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index e2d62aa..1ec5d02 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -290,6 +290,7 @@
 chromium_build_url: http://build.chromium.org/p/chromiumos/
 sheriffs: USE SHADOW SHERIFFS
 lab_sheriffs: USE SHADOW SHERIFFS
+gmail_api_credentials: USE SHADOW GMAIL_API_CREDENTIALS
 
 
 [POOL_INSTANCE_SHARDING]