Add integration tests for SSL/TLS sockets

[clue]

This PR adds a few additional integration tests for SSL/TLS sockets in the hope of addressing all related buffering issues (#14, #24, #26, #57 and possibly more).

This is essentially an updated version of #16.

[clue]

The tests are failing… *sigh* We've changed to Travis' container-based infrastructure with #54, which means executing sudo is longer allowed and installing arbitrary packages is severely limited. We need to install openssl and any (arbitrary) TLS/SSL terminating proxy.

Neither of stunnel, haproxy, stud or hitch are on the package whitelist though: https://github.com/travis-ci/apt-package-whitelist (see also travis-ci/apt-package-safelist#403)

We may look into building a TLS/SSL proxy from source, but I'd rather not go down that road.

As an alternative, I've started looking into implementing TLS/SSL socket support in React (reactphp/socket#24), which happens to works surprisingly well. However, this will likely need a few smaller patches in different React components, so expect this to take a while before this lands.

Perhaps anybody else happens to know a simple TLS/SSL proxy that is easy to set up in Travis? 👍

[clue]

We may look into building a TLS/SSL proxy from source, but I'd rather not go down that road.

Let's face it, this is probably still easier than updating our components. The PR has been updated, so let's get this in 🎉

[cboden]

Great job! Please add a milestone to this before tagging.