Custom headers

[cagataygurturk]

Custom headers support.

We should implement our own annotations to give this support due to API Gateway's limitations.

[refacktor]

+1 we need the ability to control CORS headers.

Please note there is no longer a dependency on #35, it is now possible to set custom headers in API Gateway in "normal" (non-proxy) mode.

[refacktor]

This is implemented in PR #59