Move example-gateway to a standalone example

[ChuntaoLu]

example-gateway is interleaved with Zanzibar and does not work as a standalone example. It should be a full-fledged standalone example.

[Raynos]

its both used in the integration test suite as well as being an example.

I think a full-fledged example would be a good idea; but getting the integration test suite to work against HEAD ( especially uncommitted changes ) would be non-trivial without duplicating the example twice.

[Raynos]

Whatever creative solution you come up ; we need an example that is tested on every commit of master. having an example that can be broken / stale because its not verified in travis/jenkins is more frustrating then not having an example at all.