Add examples to Travis

[Geod24]

See #548 .
Instead of putting "tests" in the CI, I think we should put everything in examples for consistency. Ifthis get pulled I'll finnish the work with "tests".

The job is done by 2 bash lines in the .travis.yml to add them, as we should rely on "dub test" doing the biggest part of the job. This may not be the best way to do it, but it's better than what we used to have. Any advice / direction is welcome.

There are some examples that are build but not run:
daytime: Address specified is not correct + use a priviledged port;
mongodb: Needs a server
redis[-pubsub]: Needs a server;
sendmail: Needs a server
I don't think it's worth installing a mongodb / redis / smtp server on the CI machine just to test the driver, so there is no attempt to run them.

[s-ludwig]

Instead of putting "tests" in the CI, I think we should put everything in examples for consistency. Ifthis get pulled I'll finnish the work with "tests".

While the "tests/" folder concept is not really finished, those do not really fit together with the "examples/" concept. The tests are intended to be run by the CI (and thus are expected to terminate with the proper error code), while the examples can run indefinitely or require user input and are not suited for execution by the CI server. It would be good to integrate the "tests" with dub test somehow, but that's all still up in the air. BTW, the two "bench_http*" "examples" should actually rather be tests, but they need to be adjusted to terminate automatically (the termination after 5000 requests isn't intended, though, a command line option to limit the number of requests may be good though).

Regarding the "echoserver" example, port 7 is the standard port for the echo protocol, so I'd rather automatically emit a warning that it must be run as root when it fails... hm, or maybe just fall back to an unprivileged port if 7 fails... can be sorted out later.

The "daytime" example works for me.

There are some overlapping changes with #551, but I'll pull this one first to have the CI integration of the examples in place. Thanks a lot for these additions/cleanups!