4 unit tests and added setters in class 'Parameter'

Unit tests for all classes in Transformer\Event that have methods.
Two setters added in Transformer\Template\Parameter
Unit test for Transformer\Template\Parameter
new unit test

Unit test for phpDocumentor\Transformer\Writer\Collection
corrections in Parameter and in two unit tests

In Parameter: changed @return Parameter into @return $this
In PreXslWriterEventTest: changed DOMDocument into DOMElement
In CollectionTest added:
- testOffsetSetWriterReceivesRouterQueue()
- testOffsetGetWithExistingIndex()
and changed testOffsetSetAndCheckRequirements() into
testCheckRequirements()
Correction in CollectionTest

Added private method 'registerWriter' for tests that need a collection
item. Used this method in two tests.
New correction in CollectionTest

Added: use phpDocumentor\Transformer\Router\Queue;
Added: use phpDocumentor\Plugin\Core\Transformer\Writer\Xsl;
Declared property $fixture
Retry because Travis CI build could not complete

Another attempt to get CollectionTest accepted by Scrutinizer
CollectionTest moved private method

Moved private function registerWriter() to the bottom of the class.
Merge pull request #1382 from mbed67/unittests20140802

4 unit tests and added setters in class 'Parameter'