pull-request: Include BaseSessionRequestHandler class in webapp2_extras.session about webapp2 HOT 1 OPEN

Thanks for this. It is a neat idea, however I don't want to add base handlers 
to webapp2. There are multiple ways that this could be done -- using mixins, 
using functions, etc, and I think this should be left to users to decide. 

Couldn't you release it as a separate helper package with extra handlers?

Side note: you can add any extra key to the configuration, and read it later. 
It doesn't need to be part of default_config.

I'll leave this issue opened because your handler may be used as example in the 
docs.