libapr alone is not sufficiant. libtcnative is required for Tomcat.

Removed X-Forwarded header handling from the application code and
documented how to setup the servlet container and reverse proxy to
handle those.

Made links relative for GitHub compatibility.

Made links relative for GitHub compatibility.

* skillquestions -> contentlists
* lecturequestions -> lecturecontentlists
* preparationquestions -> preparationcontentlists
* flashcardquestions -> flashcardcontentlists
* questions -> contents
* answers -> answerlists
* learningprogress -> score

* Prefer ids over "foreign" domain objects.
* Consistently use final keyword for implementation parameters and
  variables.
* As a consequence, session id is used instead of the object as cache
  key for `skillquestions`, `lecturequestions`, `preparationquestions`
  and`flashcardquestions` caches.
* Rename occurances of `questionId` to `contentId`.

* Moved classes from `de.thm.arsnova` root package.
* New util package
* Do not start interface names with `I`. Appended `Impl` to the
  implementing class instead.
* Renamed learning progress classes to score and move them from
  `.domain` to a new `.services.score` package.
* Do not capitalize acronyms in class names:
    * CASLogoutSuccessHandler -> CasLogoutSuccessHandler
    * SocketIO* -> Socketio*
* Renamed `.socket` package to `.websocket`

Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

The README file has been split up into separate files for installation
and development. A new documentation for the upgrade process has been
added.

The following changes have been made regarding content:
* Removed outdated Vagrant section
* Replaced unnecessary session persistance section
* Use simple HTTP proxy module instead of AJP for Apache httpd
* Nginx as alternative for Apache httpd
* Use Websocket proxy which makes HTTPS configuration on the Java side
  obsolete
* Additional configuration steps for server software
* Added requirements section
* Added upgrading instructions