Add subdirectories with nested index.html for unit tests of issue #4

See details on Github:
https://github.com/andrewrjones/ant-task-sitemap/issues/4
Add comments for two actions TODO in build.xml
Remove generated sitemaps from test-resources/ in clean task

After running one of the tasks use.* manually (these tasks are also
called in unit tests, with proper cleanup) the generated sitemaps
cause the first unit test that runs afterwards to fail.

Running 'ant clean' before 'ant test' solves this issue.

I now consider adding "clean" to the dependencies of the "test" task,
or running the tearDown() method before the first run of the unit tests
to complete this fix.
Refactoring: extract method removeGeneratedFiles() from tearDown()

The intent is to reuse the method in setUp() without calling tearDown()
directly, which would be less meaningful.
Remove generated files before as well as after running each unit tests

Generated files left from a manual run of "use.*" tasks in the build.xml
(used to) cause the following unit test to fail.