Cleanup and modularization of Testsystems package

 * Create interfaces for TestSystem, TestPage, SlimClient, ExecutionLog, Descriptor.
 * Moved (almost) all dependencies to `fitnesse.wiki` out of the `fitnesse.testsystems` package.
 * Introduced package `fitnesse.testrunner`, that contains the MultipleTestsRunner class, among support classes such as WikiTestPage and CompositeExecutionLog.
 * Test systems are themselves responsible for cleanup if an error occurred.
 * Separated the creation of a SLiM test system instance from the creation of the SlimClient.