Improving MongoDBCollectionIterator code documentation
Improve unit testing of Collection classes.

[ Enhancements ]

* The unit testing now uses parameters, to define in phpunit.xml
to define storage resources.

[ phpunit.xml configuration file ]

For FilesCollectionTest:

	* UNITTESTING_FILESCOLLECTION_PATH

For MongoDBCollectionTest:

	* UNITTESTING_MONGODB_HOST
	* UNITTESTING_MONGODB_PORT
	* UNITTESTING_MONGODB_SSL

For MySQLTest:

	* UNITTESTING_MYSQL_HOST
	* UNITTESTING_MYSQL_USERNAME
	* UNITTESTING_MYSQL_PASSWOR
	* UNITTESTING_MYSQL_DATABASE

For SQLiteCollectionTest:

	* UNITTESTING_SQLITE_FILE

[ Notes ]

* tests/phpunit.xml, like includes/config.php is the sample to ship
  and so shouldn't be committed to share your test configuration

* To run an external phpunit.xml file, you can use the command
  phpunit -c <path to your phpunit.xml> .

* MySQLTest and SQLiteCollectionTest are tests for the not yet in
  master branch MYSQL and SQLite storage collection classes.
Unit testing: adding Makefile

This simplify the way to to run tests:

    cd tests
    make test
Unit testing: tests/collections → tests/includes/collection

Tests are organized by directories, matching the code structure.