[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/.gitignore b/.gitignore
index 8b3a878..df32260 100644
--- a/.gitignore
+++ b/.gitignore
@@ -52,6 +52,7 @@
 
 # These exist on the production servers.
 bug_filing_oauth_credentials.dat
+gmail_api_credentials.dat
 site_utils/autotest_private/
 site_utils/autotest_tools/
 site_utils/devserver/
@@ -63,4 +64,4 @@
 puppylab/.vagrant
 
 # Container directory
-containers/
\ No newline at end of file
+containers/