Add get_left_id and get_right_id methods to entity class
Add tests
Fix id data key names
Update tests to use get_import_data
Merge remote-tracking branch 'upstream/master' into entity-get-tree-ids

Conflicts:
	entity/rule.php
Fix type (left -> right)
Merge pull request #29 from VSEphpbb/entity-get-tree-ids

Add get_left_id and get_right_id methods to entity class