Added grok pattern for Google App Engine logs

Hi guys,
I just added this grok pattern in order to parse Google App Engine logs. 

Here is the tool I used to download all the logs:
https://developers.google.com/appengine/docs/appcfg

There is also another option to work on logs that is based on Logs Python API (https://developers.google.com/appengine/docs/python/logs/), but the grok pattern doesn't apply in this case.