Add API Gateway IntegrationResponse header support

[elkelk]

This adds support for static and dynamic IntegrationResponse headers as specified here: http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-apitgateway-method-integration-integrationresponse.html

The PR includes a CORS example and documentation for parameters within the Integration Response section.

I dug into the tests a bit, but didn't see an easy way to insert coverage. I'd love any direction/suggestions for adding test coverage.

#68

[avengerpenguin]

+1 for this for me. Is there anything I can do to help get it in?

[ahl]

another +1 from me: I ended up implementing the same thing (here: #107), not realizing that this work had already been done. It would be great to see this accepted.

[micahwedemeyer]

👍 CORS support is super important for a huge percentage of projects that would use Lambda.

[DaRizat]

+1 Any update on when this might be available?

[YoavGivati]

+1. Would help a lot with new engineers not having to install from the src just to use this critical fix.

Starting to get concerned that it's been over 3 months since the last PR was merged in.

cc @jorgebastida

[jorgebastida]

Sorry for the long delay! This looks amazing :) Thank you all for you patience