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

Made links relative for GitHub compatibility.

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* Removed outdated Vagrant section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

One more thing that allready should have been in documentation but
drives developers and admins into madness.

Added menu for "Documentation" containing a list of dependencies, a small
indroduction how to install ARSnova for development usage and a link
to generated API docs.