PHP-1034: Ensure MongoCommandCursor implements Iterator interface properly.
Merged pull request #645
PHP-1040: MongoCommandCursor::key() should not return _id field

This also updates aggregateCursor() tests to log query/getmore requests. New tests were added for getmores due to limit/batchSize relations, as well as an error test case for invalid batchSize options.
Add missing "proto" in MongoCommandCursor methods