#55 added a method to get the CismapPlugin

Added a method to CismapUtils to directly get the instance of
CismapPlugin.
 #55 made SwitchOnConstants configurable

Made SwitchOnConstants configurable so that it can load an alternative
URL for the preview map. This URL is saved in a new configuration.xml,
this is necessary because the other XMLs will be overwritten when the
system exits.
Merge branch 'feature/60-preview-map-configurable-via-xml' into dev