Add events for lambda ALB integration

[sjurtf]

The recently added feature of ALBs was to integrate with Lambdas using a target group, and this PR is adding the events needed to this package.

I was working on integrating a Lambda with an ALB and needed to create these events to be able to write my lambda.

I've written them based on documentation page for using Lambda as ALB target.
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#respond-to-load-balancer

Edit:
This fixes parts of #146

[codecov-io]

Codecov Report

Merging #149 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   68.89%   68.89%           
=======================================
  Files          17       17           
  Lines         659      659           
=======================================
  Hits          454      454           
  Misses        165      165           
  Partials       40       40

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[qneyrat]

Can you add test and md doc ? (you can copy example https://github.com/aws/aws-lambda-go/blob/master/events/apigw_test.go#L13)

[qneyrat]

Add tests

[sjurtf]

Thank you for taking the time to reply, I'll add tests tomorrow.

[bmoffatt]

Thanks for adding the tests. I've made some cosmetic changes to the naming and updated the docs.

[furiousbunny]

Looks good to me.