Replace old API Connection with new API Client library for the Events module

> Note: this PR builds on #120 because #120 introduces several changes to the Event module. This PR is about refactoring the whole Event module to use the new API Client and use Dependency Injection.

As part of an effort to create a separate API Client as a library (so that others can benefit from it) we will have to refactor the entire application to make use of that.

This is the first PR of a series, and hence a prerequisite, where we replace existing parts of the application with this new API Client. The following actions have been taken in this PR:

1. Introduced the API Client using Composer
2. Disconnected the EventApi class from the BaseApi and inject the Event service from the Client library
3. Replaced all calls to the API in the EventAPI class with calls to the Event service
4. Removed the EventEntity class; the Event entity of the Client library is used instead
5. Moved all object instantation of the Event module to 'Service Providers'; these are classes that instantiate the services for each module. (thus there is an Application\ServiceProvider, Event\ServiceProvider and Talk\ServiceProvider)
6. Refactor all classes that use the services created in the above Service Providers to obtain them from the Slim container (`$app->container->get('service')`).
7. Wrote tests for >80% of the EventAPI class
8. Cleaned up some code

_Please note that this PR depends on **PHP 5.4**; which is not yet available in the standard VMs_

_Another note: this refactoring touches quite a bit of the Event module and a few parts in other modules, this might affect whether other PRs can be merged. I don't mind re-merging master after other PRs are merged and fixing conflicts, so it might be worth to consider merging other PRs first_