Support zooming of VNC window

This is a feature which was often required by users who use notebooks
with a resolution which is than the resolution of the PalMA team monitor.

When zooming the VNC window, it will be centered on the screen, so it
is necessary to select the display layout after zooming to get
the correct layout again.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Update installation documentation

* The web browser midori was replaced by dwb because Debian Jessie no
  longer includes a midori package.

* Large file uploads need an additional setting in /etc/php5/apache2/php.ini.
  Both upload_max_filesize and post_max_size must be set to the desired
  value.

Signed-off-by: Stefan Weil <sw@weilnetz.de>