Use standard JDK logging configuration

Hi Matthias,

I have refactored the LogPolicy into a JDK logging Formatter class which can be configured in a logging configuration file. I have included an example file (under src/test/resources) which is already used for running the unit tests (see pom.xml). When using this approach for a standalone server, the VM parameter for configuring the logging configuration file needs to be provided on the command line. Using this approach it also doesn't make much sense for all the other classes to refer to CaliforniumLogger in order to get a logger. Instead, they should simply use the standard JDK LogManager.getLogger() method ... all configuration can be done using the properties file ...