Release Candidate: major design overhaul, more TLS options, quality improvements and extensive examples

With this release, long awaited features are finally here. Not only is there now full support for all TLS options offered and supported by PHP SSL contexts. Also, a lot of work has been put into refactoring and testing of existing code. A few APIs have been moved and renamed, and there is now also an extensive collection of examples available in the php-mqtt/client-examples repository.

A (preliminary) full list of changes can be found in the CHANGELOG.md.

If you have the time, please upgrade your application to this version and give feedback how it works for you. In about a week, a final version v1.0.0 will be published if everything goes by plan.