Added support for conditionally disabling tests that require an active internet connection

Some of the tests require an active internet connection and access to www.thomas-bayer.com . Our build server does not allow arbitrary outgoing connections.

To still allow building and testing the core module I added support to ignore the tests that require a connection to www.thomas-bayer.com at runtime by adding an environment variable OFFLINETESTING with a non empty value.

Using org.junit.Assume did require to make those tests jUnit 4 style, using the test annotations and not extending GroovyTestCase.