Revert "Merge pull request #281 from MartinGijsen/master"

This reverts commit 3a943a81fd18eb7113ebb5fa4cd82aa8f624b11b, reversing
changes made to 8222ce91ba0d46fdd818836d73a79afdd58cc3f3.
Get rid of fastTest property in TestSystemGroup.

It’s added to WikiPageDecorator to create specific test system
names for in process and out process runs.
Remove dependency on FitNesseContext for FitTestSystem.
Move test system creation to separate factory. Clean up TestSystemFactory interface.

This commit is a little big:
 * Remove Listener interface from TestSystemFactory.
 * Remove TestSystemGroup: test suites are executed in a linear fashion.
Inject TestSystemFactory in MultipleTestRunner.
Updated unit tests for MultipleTestsRunner.

Looks like the responsibilities changed somewhat.
Explicitly set TestingTracker on MultipleTestsRunner.

Still need to get rid of the FitNesseContext dependency.
Move page-by-test-system ordering to a separate class.

Executing tests and grouping tests are two separate concerns.
Little cleanup in ClassPathBuilder.
Externalise creation of Descriptor outside of PagesByTestSystem algorithm.
Let PagesByTestSystem and the surrounder class work with just wiki pages.

No need to wrap as TestPage already.
Make page order resolving logic somewhat more functional.

Do less (mis)use of state variables.
Merge branch 'master' into HEAD
Can load test systems via plugins loader.
Generalised common key:className code.
Hook up test systems loading to main app.
Remove unused method.
Log failed calls to page responders.
Fixed 368: External suite pages not working.
FitNesse used text/plain as the content type for images.